Update to the 20180104 snapshot of GCC 8.2.1.
Forward port r469788 from lang/gcc6 = backport r475105 from lang/gcc8:
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.
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