Several CPU features require other ones--for example, AVX requires the XSAVE feature. Additionally, x86-64 implicitly requires several features (most notably SSE2), and the glibc folks have been working on a proposed ABI for x86-64 that groups the feature sets into levels--roughly base (≤SSE2), ≤SSE4.2, ≤AVX2, current skylake-server (a clutch of AVX-512 features), with the BMI and FMA features scattered in there somewhere.
That said, the MMX instructions in particular are so problematic to use (and SSE ubiquitous and strictly better) that I suspect you could introduce a processor that lacks MMX support and break almost nobody, certainly far fewer people than removing x87. I don't know if there is an implicit or explicit actual dependency on MMX anywhere.
We can't really demand compilers to create code that's compatible with all the ancient variations of a currently popular architecture. It's still good manners to include a function that exits with a clear error message about a required architecture feature that's missing.
I had similar issues with PPC software that just blindly assumed I had Altivec on my G3. It wasn't fun.
