Features: Extend concept to C language.

Add properties and variables corresponding to CXX equivalents.

Add features for c_function_prototypes (C90), c_restrict (C99),
c_variadic_macros (C99) and c_static_assert (C11). This feature
set can be extended later.

Add a <PREFIX>_RESTRICT symbol define to WriteCompilerDetectionHeader
to conditionally represent the c_restrict feature.

12 files changed, 122 insertions, 1 deletions

diff --git a/Help/command/target_compile_features.rst b/Help/command/target_compile_features.rst
index f8e5c54..9559600 100644
--- a/Help/command/target_compile_features.rst
+++ b/Help/command/target_compile_features.rst
@@ -8,7 +8,8 @@ Add expected compiler features to a target.
   target_compile_features(<target> <PRIVATE|PUBLIC|INTERFACE> <feature> [...])
 
 Specify compiler features required when compiling a given target.  If the
-feature is not listed in the :variable:`CMAKE_CXX_COMPILE_FEATURES` variable,
+feature is not listed in the :variable:`CMAKE_C_COMPILE_FEATURES` variable
+or :variable:`CMAKE_CXX_COMPILE_FEATURES` variable,
 then an error will be reported by CMake.  If the use of the feature requires
 an additional compiler flag, such as ``-std=c++11``, the flag will be added
 automatically.
diff --git a/Help/manual/cmake-properties.7.rst b/Help/manual/cmake-properties.7.rst
index 14d2e7f..a204a9c 100644
--- a/Help/manual/cmake-properties.7.rst
+++ b/Help/manual/cmake-properties.7.rst
@@ -16,6 +16,7 @@ Properties of Global Scope
    /prop_gbl/ALLOW_DUPLICATE_CUSTOM_TARGETS
    /prop_gbl/AUTOGEN_TARGETS_FOLDER
    /prop_gbl/AUTOMOC_TARGETS_FOLDER
+   /prop_gbl/CMAKE_C_KNOWN_FEATURES
    /prop_gbl/CMAKE_CXX_KNOWN_FEATURES
    /prop_gbl/DEBUG_CONFIGURATIONS
    /prop_gbl/DISABLED_FEATURES
@@ -93,6 +94,9 @@ Properties on Targets
    /prop_tgt/BUILD_WITH_INSTALL_RPATH
    /prop_tgt/BUNDLE_EXTENSION
    /prop_tgt/BUNDLE
+   /prop_tgt/C_EXTENSIONS
+   /prop_tgt/C_STANDARD
+   /prop_tgt/C_STANDARD_REQUIRED
    /prop_tgt/COMPATIBLE_INTERFACE_BOOL
    /prop_tgt/COMPATIBLE_INTERFACE_NUMBER_MAX
    /prop_tgt/COMPATIBLE_INTERFACE_NUMBER_MIN
diff --git a/Help/manual/cmake-variables.7.rst b/Help/manual/cmake-variables.7.rst
index a7d4af6..b2dc88f 100644
--- a/Help/manual/cmake-variables.7.rst
+++ b/Help/manual/cmake-variables.7.rst
@@ -260,6 +260,10 @@ Variables for Languages
    :maxdepth: 1
 
    /variable/CMAKE_COMPILER_IS_GNULANG
+   /variable/CMAKE_C_COMPILE_FEATURES
+   /variable/CMAKE_C_EXTENSIONS
+   /variable/CMAKE_C_STANDARD
+   /variable/CMAKE_C_STANDARD_REQUIRED
    /variable/CMAKE_CXX_COMPILE_FEATURES
    /variable/CMAKE_CXX_EXTENSIONS
    /variable/CMAKE_CXX_STANDARD
diff --git a/Help/prop_gbl/CMAKE_C_KNOWN_FEATURES.rst b/Help/prop_gbl/CMAKE_C_KNOWN_FEATURES.rst
new file mode 100644
index 0000000..c57bc73
--- /dev/null
+++ b/Help/prop_gbl/CMAKE_C_KNOWN_FEATURES.rst
@@ -0,0 +1,25 @@
+CMAKE_C_KNOWN_FEATURES
+----------------------
+
+List of C features known to this version of CMake.
+
+The features listed in this global property may be known to be available to the
+C compiler.  If the feature is available with the C compiler, it will
+be listed in the :variable:`CMAKE_C_COMPILE_FEATURES` variable.
+
+The features listed here may be used with the :command:`target_compile_features`
+command.
+
+The features known to this version of CMake are:
+
+``c_function_prototypes``
+  Function prototypes, as defined in ``ISO/IEC 9899:1990``.
+
+``c_restrict``
+  ``restrict`` keyword, as defined in ``ISO/IEC 9899:1999``.
+
+``c_static_assert``
+  Static assert, as defined in ``ISO/IEC 9899:2011``.
+
+``c_variadic_macros``
+  Variadic macros, as defined in ``ISO/IEC 9899:1999``.
diff --git a/Help/prop_tgt/C_EXTENSIONS.rst b/Help/prop_tgt/C_EXTENSIONS.rst
new file mode 100644
index 0000000..246e93d
--- /dev/null
+++ b/Help/prop_tgt/C_EXTENSIONS.rst
@@ -0,0 +1,8 @@
+C_EXTENSIONS
+------------
+
+Boolean specifying whether compiler specific extensions are requested.
+
+This property specifies whether compiler specific extensions should be
+used.  For some compilers, this results in adding a flag such
+as ``-std=gnu11`` instead of ``-std=c11`` to the compile line.
diff --git a/Help/prop_tgt/C_STANDARD.rst b/Help/prop_tgt/C_STANDARD.rst
new file mode 100644
index 0000000..9fdc0bb
--- /dev/null
+++ b/Help/prop_tgt/C_STANDARD.rst
@@ -0,0 +1,27 @@
+C_STANDARD
+----------
+
+The C standard whose features are requested to build this target.
+
+This property specifies the C standard whose features are requested
+to build this target.  For some compilers, this results in adding a
+flag such as ``-std=c11`` to the compile line.
+
+Supported values are ``90``, ``99`` and ``11``.
+
+If the value requested does not result in a compile flag being added for
+the compiler in use, a previous standard flag will be added instead.  This
+means that using:
+
+.. code-block:: cmake
+
+  set_property(TARGET tgt PROPERTY C_STANDARD 11)
+
+with a compiler which does not support ``-std=c11`` or an equivalent
+flag will not result in an error or warning, but will instead add the
+``-std=c99`` or ``-std=c90`` flag if supported.  This "decay" behavior may
+be controlled with the :prop_tgt:`C_STANDARD_REQUIRED` target property.
+
+This property is initialized by the value of
+the :variable:`CMAKE_C_STANDARD` variable if it is set when a target
+is created.
diff --git a/Help/prop_tgt/C_STANDARD_REQUIRED.rst b/Help/prop_tgt/C_STANDARD_REQUIRED.rst
new file mode 100644
index 0000000..6c39e96
--- /dev/null
+++ b/Help/prop_tgt/C_STANDARD_REQUIRED.rst
@@ -0,0 +1,14 @@
+C_STANDARD_REQUIRED
+-------------------
+
+Boolean describing whether the value of :prop_tgt:`C_STANDARD` is a requirement.
+
+If this property is set to ``ON``, then the value of the
+:prop_tgt:`C_STANDARD` target property is treated as a requirement.  If this
+property is ``OFF`` or unset, the :prop_tgt:`C_STANDARD` target property is
+treated as optional and may "decay" to a previous standard if the requested is
+not available.
+
+This property is initialized by the value of
+the :variable:`CMAKE_C_STANDARD_REQUIRED` variable if it is set when a
+target is created.
diff --git a/Help/release/dev/compile-language-features.rst b/Help/release/dev/compile-language-features.rst
index 3c5d7ca..fe72e39 100644
--- a/Help/release/dev/compile-language-features.rst
+++ b/Help/release/dev/compile-language-features.rst
@@ -7,6 +7,12 @@ target-language-features
   :variable:`CMAKE_CXX_STANDARD` and :variable:`CMAKE_CXX_EXTENSIONS`
   variables may be set to initialize the target properties.
 
+* New :prop_tgt:`C_STANDARD` and :prop_tgt:`C_EXTENSIONS` target
+  properties may specify values which CMake uses to compute required
+  compile options such as ``-std=c11`` or ``-std=gnu11``. The
+  :variable:`CMAKE_C_STANDARD` and :variable:`CMAKE_C_EXTENSIONS`
+  variables may be set to initialize the target properties.
+
 * New :prop_tgt:`COMPILE_FEATURES` target property may contain a list
   of features required to compile a target.  CMake uses this
   information to ensure that the compiler in use is capable of building
diff --git a/Help/variable/CMAKE_C_COMPILE_FEATURES.rst b/Help/variable/CMAKE_C_COMPILE_FEATURES.rst
new file mode 100644
index 0000000..7d1c8b1
--- /dev/null
+++ b/Help/variable/CMAKE_C_COMPILE_FEATURES.rst
@@ -0,0 +1,8 @@
+CMAKE_C_COMPILE_FEATURES
+------------------------
+
+List of features known to the C compiler
+
+These features are known to be available for use with the C compiler. This
+list is a subset of the features listed in the :prop_gbl:`CMAKE_C_KNOWN_FEATURES`
+global property.
diff --git a/Help/variable/CMAKE_C_EXTENSIONS.rst b/Help/variable/CMAKE_C_EXTENSIONS.rst
new file mode 100644
index 0000000..ff2569b
--- /dev/null
+++ b/Help/variable/CMAKE_C_EXTENSIONS.rst
@@ -0,0 +1,8 @@
+CMAKE_C_EXTENSIONS
+------------------
+
+Default value for ``C_EXTENSIONS`` property of targets.
+
+This variable is used to initialize the :prop_tgt:`C_EXTENSIONS`
+property on all targets.  See that target property for additional
+information.
diff --git a/Help/variable/CMAKE_C_STANDARD.rst b/Help/variable/CMAKE_C_STANDARD.rst
new file mode 100644
index 0000000..c1f8c29
--- /dev/null
+++ b/Help/variable/CMAKE_C_STANDARD.rst
@@ -0,0 +1,8 @@
+CMAKE_C_STANDARD
+----------------
+
+Default value for ``C_STANDARD`` property of targets.
+
+This variable is used to initialize the :prop_tgt:`C_STANDARD`
+property on all targets.  See that target property for additional
+information.
diff --git a/Help/variable/CMAKE_C_STANDARD_REQUIRED.rst b/Help/variable/CMAKE_C_STANDARD_REQUIRED.rst
new file mode 100644
index 0000000..13ea49f
--- /dev/null
+++ b/Help/variable/CMAKE_C_STANDARD_REQUIRED.rst
@@ -0,0 +1,8 @@
+CMAKE_C_STANDARD_REQUIRED
+-------------------------
+
+Default value for ``C_STANDARD_REQUIRED`` property of targets.
+
+This variable is used to initialize the :prop_tgt:`C_STANDARD_REQUIRED`
+property on all targets.  See that target property for additional
+information.