Add target_compile_features command.

This can be used to set the compiler features required by particular
targets. An error is issued at CMake time if the compiler does not
support the required feature. If a language dialect flag is required
by the features used, that will be added automatically.

Base the target_compile_features command on cmTargetPropCommandBase. This
gives us 'free' handling of IMPORTED, ALIAS, INTERFACE, non-compilable
and missing targets.

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diff --git a/Help/command/target_compile_features.rst b/Help/command/target_compile_features.rst
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+target_compile_features
+-----------------------
+
+Add expected compiler features to a target.
+
+::
+
+  target_compile_features(<target> <PRIVATE|PUBLIC|INTERFACE> <feature> [...])
+
+Specify compiler features required when compiling a given target.  If the
+feature is not listed in the :variable:`CMAKE_CXX_COMPILE_FEATURES` variable,
+then an error will be reported by CMake.  If the use of the feature requires
+an additional compiler flag, such as ``-std=c++11``, the flag will be added
+automatically.
+
+The ``INTERFACE``, ``PUBLIC`` and ``PRIVATE`` keywords are required to
+specify the scope of the features.  ``PRIVATE`` and ``PUBLIC`` items will
+populate the :prop_tgt:`COMPILE_FEATURES` property of ``<target>``.
+``PUBLIC`` and ``INTERFACE`` items will populate the
+:prop_tgt:`INTERFACE_COMPILE_FEATURES` property of ``<target>``.  Repeated
+calls for the same ``<target>`` append items.
+
+The named ``<target>`` must have been created by a command such as
+:command:`add_executable` or :command:`add_library` and must not be
+an ``IMPORTED`` target.
+
+Arguments to ``target_compile_features`` may use "generator expressions"
+with the syntax ``$<...>``.
+See the :manual:`cmake-generator-expressions(7)` manual for available
+expressions.