This port tries to use SSE instrinsics. Add to CXXFLAGS on powerpc64 -DNO_WARN_X86_INTRINSICS -mvsx, which enable using SSE to Altivec translation layer. Because of that, also use new GCC. Note that intrinsics translations needs VSX, which is only supported on POWER8 and newer, but this port doesn't currently work on powerpc64 anyway.
Limit -Wno-narrowing to only clang, gcc doesn't support it.
Also add USES=gl for PLUGINS option.