project: Add infrastructure for recording CXX compiler features

Add a feature test using the compiler macros and the preprocessor to
determine available features.

Add a CMAKE_CXX_COMPILE_FEATURES variable which contains all features
known to the loaded compiler, and a CMAKE_CXX_KNOWN_FEATURES variable
containing all features known to CMake.  Add language standard specific
variables for internal use to determine the standard-specific compile
flags to use.

This will be extended to other languages in the future.  Follow-up
commits will add features which will be recorded by the feature test.

Help/manual/cmake-variables.7.rst

@@ -257,6 +257,8 @@ Variables for Languages
    :maxdepth: 1
 
    /variable/CMAKE_COMPILER_IS_GNULANG
+   /variable/CMAKE_CXX_COMPILE_FEATURES
+   /variable/CMAKE_CXX_KNOWN_FEATURES
    /variable/CMAKE_CXX_STANDARD
    /variable/CMAKE_CXX_EXTENSIONS
    /variable/CMAKE_Fortran_MODDIR_DEFAULT

Help/variable/CMAKE_CXX_COMPILE_FEATURES.rst

@@ -0,0 +1,8 @@
+CMAKE_CXX_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 :variable:`CMAKE_CXX_KNOWN_FEATURES`
+variable.

Help/variable/CMAKE_CXX_KNOWN_FEATURES.rst

@@ -0,0 +1,8 @@
+CMAKE_CXX_KNOWN_FEATURES
+------------------------
+
+List of C++ features known to this version of CMake.
+
+The features listed in this variable 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_CXX_COMPILE_FEATURES` variable.