If someone only includes xmmintrin.h, but references any definitions from stdlib.h (included via mm_malloc.h), if module are enabled it will fail, but if modules are not enable, it will not fail to compile. This will fix this (if STDC_HOSTED is defined).
Did you intentionally remove this comment?
You shouldn't need to add anything here. sse2 exports sse, which in turn exports mm_malloc and mmx, so users of sse2 get mm_malloc and mmx whether or not sse2 lists them. It's better for it to not do so, as they aren't direct dependencies.