Filter the -mretpoline command-line option, which is understood by
recent versions of clang (and used in the context of the Spectre
security issues), but not GCC (which uses different options for the
same) from CFLAGS and CXXFLAGS.

This avoids the build of this port via bootstrap, which is the default
and leverages both the system compiler (clang in most cases) plus a just
built version GCC, to fail due to the latter not knowing -mretpoline.