Fix ParseFlags so -stdlib flag goes in CXXFLAGS

[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

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

[mwichmann]

Yes, unfortunately ParseFlags depends on being able to recognize each option, so it's down to people figuring out something is needed and how to handle it. Looks reasonable to me.

Will need to work up changleog entries and preferably add a testcase (most of these are touched either in SCons/EnvironmentTests.py or in test/ParseConfig.py).

[bdbaddog]

@ryandesign - Yup as @mwichmann , please add a blurb to CHANGES.txt and RELEASE.txt, and a test and we'll get this merged.

[ryandesign]

I'm not planning any further changes to this PR right now but others are welcome to add the missing pieces.

[mwichmann]

Closing this in favor of #4521 which should complete the process (it was easier to spin up a new PR with small changes to four additional files than have to modify the existing one)