multimedia/obs-qtwebkit: fix build on powerpc64

cc1plus: error: unrecognized command line option "-std=c++11"

Additionally, this port tries to use SSE, so it needs -DNO_WARN_X86_INTRINSICS -maltivec -mvsx on powerpc64 to use GCC's translation layer to AltiVec.

Also respect CXXFLAGS to make it work.

PR: 243658
Approved by: swills (maintainer)
MFH: 2020q2 (fix build blanket)