Fixed bug 2241 - SSE intrinsic in fillrect MMX path

norfanin

The MMX path in SDL_fillrect.c uses the SSE intrinsic _mm_stream_pi. The function or symbol provided by the compiler will not be present because the SSE header may not get included. The linker will complain about an undefined reference.

Since this is the only intrinsic used here (and someone forgot to create one for MOVQ), I think the MMX path can be removed completely. At least I don't see another way to move 64-bits from an MMX register to memory.
Fixed bug 2238 - Enable GCC atomics for clang

Marcus von Appen

clang provides support for optimized atomics.
The attached patch enables the cmake build system to take clang into account on checking for atomics.