Fix ParseFlags so -stdlib goes in CXXFLAGS

[mwichmann]

(from #4520 @ryandesign)

gpsd 3.25 failed to build with recent clang if -stdlib=libc++ was passed by the user in CXXFLAGS because gpsd used MergeFlags (which uses ParseFlags) to move the flags where SCons thinks they belong, and because SCons did not know where -stdlib goes it put it in CCFLAGS, which was then passed to the C compiler during a test, and clang emitted a warning that the -stdlib flag was unknown, and the test had requested that warnings be turned into errors with -Werror, which caused the test to get the wrong result which caused compilation to fail later.

See: https://gitlab.com/gpsd/gpsd/-/issues/279

It was easier to make a new PR with the additional files to update. This obsoletes PR #4520

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt (and read the README.rst)
• I have updated the appropriate documentation