This allows including C headers from C++. It sadly reduced the
diagnostics on C++ code as well, were there warning may make sense as
unintended side effect. We may be able to drop that later on, when more
C APIs are properly wrapped in native C++ APIs, so that C headers do no
longer need to be compatible with C++ compilers.
Makefiles don't do comments, so these were forwarded into the variable.
*Most* users would expand the arguments to a shell where it'd be
ignored, but not all of them.
Contributes-To: https://github.com/RIOT-OS/RIOT/pull/18489
(This is also where the one version that is added here was removed).
