Document CMAKE_<LANG>_FLAGS variable (#14305)

We already document the variables CMAKE_<LANG>_FLAGS_{DEBUG, MINSIZEREL,
RELEASE, RELWITHDEBINFO}. However, CMAKE_<LANG>_FLAGS is not documented,
even though it's available (and useful).
target_link_libraries: Add PUBLIC/PRIVATE/INTERFACE keyword signature

Add a new signature to help populate INTERFACE_LINK_LIBRARIES and
LINK_LIBRARIES cleanly in a single call.  Add policy CMP0023 to control
whether the keyword signatures can be mixed with uses of the plain
signatures on the same target.
FindQt4: Re-add QAxServer to the QT_MODULES.

This was removed instead of moved in commit b8af46cb (FindQt4: Don't
fail if certain Qt modules are unavailable.).
Test COMPILE_DEFINITIONS target property get/set/get round-trip

Teach the Preprocess test to get, set, and then get the same value for
the COMPILE_DEFINITIONS target property and verify that the value is not
changed.  This ensures the internal structured storage of the property
value can reproduce the original string value.
Merge topic 'doc-CMAKE_lang_FLAGS'

41bb95f Document CMAKE_<LANG>_FLAGS variable (#14305)
Merge topic 'fix-FindQt4-available-modules'

d57c1c1 FindQt4: Re-add QAxServer to the QT_MODULES.
Merge topic 'compile-defs-debugging'

a43e5e0 Test COMPILE_DEFINITIONS target property get/set/get round-trip
Merge topic 'tll-new-signatures'

b655865 target_link_libraries: Add PUBLIC/PRIVATE/INTERFACE keyword signature