reenable detection of AVX capabilities even if compiler would not generate vector code

[abouteiller]

Reenable detection of AVX capabilities even if compiler would not generate vectorized AVX code (because libraries like mkl can still use it). With the current status, the AVX availability on the CPU is never detected because we don't pass the necessary -march=xyz that enables AVX instruction generation.

@bosilca you made the opposite change some time ago, and I have vague recollection that there were issues with compiling the builtin checks without the ifdefs on some systems?