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>

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.

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.