This commit fixes the cmparseMSBuildXML.py script to generate correct
flag table entries for booleans with optional value.  These flags use
two entries: the first should ignore the value and enable the option,
and the second should use the value if given.  Previously the first
entry did not recognize flags with values.

In particular this fixes flags like /MP4, but the change corrects
matching of some other flags too.  See issue #9771.