Fix clang version check and remove clang 12 warning
Fix clang version check and remove clang 12 warning

The clang version check in build/MakefileWorker.mk only worked for apple
clang, so this changes the version checking to work with apple, debian,
and ubuntu clang version strings.

Additionally, clang 12 added -Wpoison-system-directories, which warns if
you if you include stuff from /usr/local/include when doing a cross
compile.
Apple clang thinks it's always doing a cross compile for some reason, so
that always complains.

Closes #1456

offa/cpputest,basvodde/cpputest,offa/cpputest,offa/cpputest,cpputest/cpputest,cpputest/cpputest,cpputest/cpputest,offa/cpputest,basvodde/cpputest,basvodde/cpputest,basvodde/cpputest,cpputest/cpputest