Replace the cmLocalGenerator GetCompileOptions method with an
AddCompileOptions method since all call sites of the former simply
append the result to a flags string anyway.

Add a "lang" argument to AddCompileOptions and move the
CMAKE_<LANG>_FLAGS_REGEX filter into it.  Move the call sites in each
generator to a location that has both the language and configuration
available.  In the Makefile generator this also moves the flags from
build.make to flags.make where they belong.