Enable optional executable suffix to indicate SIMD level

[mr-c]

Hello!

Debian uses this patch as we compile hh-suite multiple times on x86-(64), one at each SIMD level.

As an alternative, hh-suite could implement runtime CPU dispatching

[milot-mirdita]

Could you add a line similar to set(CHECK_MPI 1 CACHE BOOL "Check MPI availability") etc?

[mr-c]

@milot-mirdita If EXE_SUFFIX isn't defined when calling cmake -DEXE_SUFFIX=ssse3 then it is ignored. Is that good enough?

[milot-mirdita]

I like doing that since it also contains this short comment that explains what the variable does.

[mr-c]

@milot-mirdita Ah, I understand now. Done!

[milot-mirdita]

I'd recommend to build only SSE2 and AVX2 since we don't use any instructions between those versions and the binaries get quite large.

Thanks a lot!