Do not pass -mmmx or -msse2 to the compiler
x86 GCC does not need those parameters to compile MMX or SSE assembly or built-in intrinsics (contrary to ARM GCC w.r.t. NEON). They only allow the compiler to issue MMX or SSE instructions for the plain C code. We already rely on this tolerant compiler semantic for the CPU detection code, and for some optionally accelerated filters (e.g. deinterlace). Disabling MMX and SSE for non-assembly code should have no or negligible effects on the affected plugins. On the other hand, it ensures that the plugin descriptor can be run by non-MMX/non-SSE CPUs.