Unset is needed given CC is not expected to be a cross-compiler
but, if users know what they are doing, leave a way (MINGW_BYPASS=1).
If e.g. llvm-mingw is ever officially supported, may need a mingw
eclass to juggle toolchains properly and instruct about them.
Signed-off-by: Ionen Wolkens <ionen@gentoo.org>