cmTarget: Add COMPILE_FEATURES target property.

Use the contents of it to upgrade the CXX_STANDARD target property,
if appropriate.  This will have the effect of adding the -std=c++11
compile flag or other language specification on GNU when that is
needed for the feature.

Help/manual/cmake-properties.7.rst

@@ -98,6 +98,7 @@ Properties on Targets
    /prop_tgt/COMPATIBLE_INTERFACE_STRING
    /prop_tgt/COMPILE_DEFINITIONS_CONFIG
    /prop_tgt/COMPILE_DEFINITIONS
+   /prop_tgt/COMPILE_FEATURES
    /prop_tgt/COMPILE_FLAGS
    /prop_tgt/COMPILE_OPTIONS
    /prop_tgt/COMPILE_PDB_NAME

Help/prop_tgt/COMPILE_FEATURES.rst

@@ -0,0 +1,7 @@
+COMPILE_FEATURES
+----------------
+
+Compiler features enabled for this target.
+
+The list of features in this property are a subset of the features listed
+in the :variable:`CMAKE_CXX_COMPILE_FEATURES` variable.

Help/release/dev/compile-language-features.rst

@@ -6,3 +6,9 @@ target-language-features
   compile options such as ``-std=c++11`` or ``-std=gnu++11``. The
   :variable:`CMAKE_CXX_STANDARD` and :variable:`CMAKE_CXX_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
+  the target, and to add any necessary compile flags to support language
+  features.