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.

3 files changed, 49 insertions, 0 deletions

diff --git a/Help/prop_tgt/C_EXTENSIONS.rst b/Help/prop_tgt/C_EXTENSIONS.rst
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+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
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+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
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+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.