Languages: Add support for Objective-C

Add entries in Modules and Modules/Platform to support
Objective-C compiler determination and identification.
Add Modules to check Objective-C compiler flags, source
compilations, program checks, etc...

Use OBJC as the designator of the language, eg:

project(foo OBJC)

Add various tests for Objective-C language features.  Add
tests to preserve C++ handling of .m and .mm files when
OBJC is not a configured language.

Co-Authored-By: Cristian Adam <cristian.adam@gmail.com>

7 files changed, 18 insertions, 3 deletions

diff --git a/Help/command/enable_language.rst b/Help/command/enable_language.rst
index fb49b44..44beb54 100644
--- a/Help/command/enable_language.rst
+++ b/Help/command/enable_language.rst
@@ -1,7 +1,7 @@
 enable_language
 ---------------
 
-Enable a language (CXX/C/Fortran/etc)
+Enable a language (CXX/C/OBJC/Fortran/etc)
 
 .. code-block:: cmake
 
@@ -10,7 +10,7 @@ Enable a language (CXX/C/Fortran/etc)
 Enables support for the named language in CMake.  This is
 the same as the :command:`project` command but does not create any of the extra
 variables that are created by the project command.  Example languages
-are ``CXX``, ``C``, ``CUDA``, ``Fortran``, and ``ASM``.
+are ``CXX``, ``C``, ``CUDA``, ``OBJC``, ``Fortran``, and ``ASM``.
 
 If enabling ``ASM``, enable it last so that CMake can check whether
 compilers for other languages like ``C`` work for assembly too.
diff --git a/Help/command/project.rst b/Help/command/project.rst
index baf18be..2bbb3f4 100644
--- a/Help/command/project.rst
+++ b/Help/command/project.rst
@@ -87,7 +87,8 @@ The options are:
   Can also be specified without ``LANGUAGES`` keyword per the first, short signature.
 
   Selects which programming languages are needed to build the project.
-  Supported languages include ``C``, ``CXX`` (i.e.  C++), ``CUDA``, ``Fortran``, and ``ASM``.
+  Supported languages include ``C``, ``CXX`` (i.e.  C++), ``CUDA``, ``OBJC`` (i.e. Objective-C),
+  ``Fortran``, and ``ASM``.
   By default ``C`` and ``CXX`` are enabled if no language options are given.
   Specify language ``NONE``, or use the ``LANGUAGES`` keyword and list no languages,
   to skip enabling any languages.
diff --git a/Help/manual/cmake-generator-expressions.7.rst b/Help/manual/cmake-generator-expressions.7.rst
index c0449fb..8890433 100644
--- a/Help/manual/cmake-generator-expressions.7.rst
+++ b/Help/manual/cmake-generator-expressions.7.rst
@@ -130,6 +130,11 @@ Variable Queries
   ``1`` if the CMake's compiler id of the CUDA compiler matches any one
   of the entries in ``compiler_ids``, otherwise ``0``.
   See also the :variable:`CMAKE_<LANG>_COMPILER_ID` variable.
+``$<OBJC_COMPILER_ID:compiler_ids>``
+  where ``compiler_ids`` is a comma-separated list.
+  ``1`` if the CMake's compiler id of the Objective-C compiler matches any one
+  of the entries in ``compiler_ids``, otherwise ``0``.
+  See also the :variable:`CMAKE_<LANG>_COMPILER_ID` variable.
 ``$<Fortran_COMPILER_ID:compiler_ids>``
   where ``compiler_ids`` is a comma-separated list.
   ``1`` if the CMake's compiler id of the Fortran compiler matches any one
@@ -413,6 +418,9 @@ Variable Queries
 ``$<CUDA_COMPILER_VERSION>``
   The version of the CUDA compiler used.
   See also the :variable:`CMAKE_<LANG>_COMPILER_VERSION` variable.
+``$<OBJC_COMPILER_VERSION>``
+  The version of the OBJC compiler used.
+  See also the :variable:`CMAKE_<LANG>_COMPILER_VERSION` variable.
 ``$<Fortran_COMPILER_VERSION>``
   The version of the Fortran compiler used.
   See also the :variable:`CMAKE_<LANG>_COMPILER_VERSION` variable.
diff --git a/Help/manual/cmake-modules.7.rst b/Help/manual/cmake-modules.7.rst
index fc4bfdc..78881e6 100644
--- a/Help/manual/cmake-modules.7.rst
+++ b/Help/manual/cmake-modules.7.rst
@@ -36,6 +36,9 @@ These modules are loaded using the :command:`include` command.
    /module/CheckIncludeFiles
    /module/CheckLanguage
    /module/CheckLibraryExists
+   /module/CheckOBJCCompilerFlag
+   /module/CheckOBJCSourceCompiles
+   /module/CheckOBJCSourceRuns
    /module/CheckPIESupported
    /module/CheckPrototypeDefinition
    /module/CheckStructHasMember
diff --git a/Help/module/CheckOBJCCompilerFlag.rst b/Help/module/CheckOBJCCompilerFlag.rst
new file mode 100644
index 0000000..e4bd6fd
--- /dev/null
+++ b/Help/module/CheckOBJCCompilerFlag.rst
@@ -0,0 +1 @@
+.. cmake-module:: ../../Modules/CheckOBJCCompilerFlag.cmake
diff --git a/Help/module/CheckOBJCSourceCompiles.rst b/Help/module/CheckOBJCSourceCompiles.rst
new file mode 100644
index 0000000..d4a1484
--- /dev/null
+++ b/Help/module/CheckOBJCSourceCompiles.rst
@@ -0,0 +1 @@
+.. cmake-module:: ../../Modules/CheckOBJCSourceCompiles.cmake
diff --git a/Help/module/CheckOBJCSourceRuns.rst b/Help/module/CheckOBJCSourceRuns.rst
new file mode 100644
index 0000000..c72f0db
--- /dev/null
+++ b/Help/module/CheckOBJCSourceRuns.rst
@@ -0,0 +1 @@
+.. cmake-module:: ../../Modules/CheckOBJCSourceRuns.cmake