Add generator expression support to per-source COMPILE_FLAGS

This allows users to specify different genex-based compile flags for
each file in a target, e.g. compiling just a single file with `Od/Ox` in
release builds on Visual Studio.

Help/prop_sf/COMPILE_FLAGS.rst

@@ -6,3 +6,10 @@ Additional flags to be added when compiling this source file.
 These flags will be added to the list of compile flags when this
 source file builds.  Use :prop_sf:`COMPILE_DEFINITIONS` to pass
 additional preprocessor definitions.
+
+Contents of ``COMPILE_FLAGS`` may use "generator expressions"
+with the syntax ``$<...>``.  See the :manual:`cmake-generator-expressions(7)`
+manual for available expressions.  However, :generator:`Xcode`
+does not support per-config per-source settings, so expressions
+that depend on the build configuration are not allowed with that
+generator.

Help/release/dev/src-COMPILE_FLAGS-genex.rst

@@ -0,0 +1,5 @@
+src-COMPILE_FLAGS-genex
+-----------------------
+
+* The :prop_sf:`COMPILE_FLAGS` source file property learned to support
+  :manual:`generator expressions <cmake-generator-expressions(7)>`.