SNAP from gcc-pkgconfig builds fail with SIGILL on older machines without avx2/fma

[heikoklein]

SNAP build with the default gcc_pkgconfig.mk automatically enforce avx2 and fma. These instructions have been available since about 2013 and are therefore generally widely available. It should be noted that Pentium and Celeron processors didn't get these instructions before 2020. Other examples without these instructions are some cloud-platforms (ostack/default bionic images) or ppi-blogin machines. If run on any of these unsupported machines, SNAP will fail with a SIGILL (illegal instructions signal) in strange places.

There are large performance benefits running SNAP with these instruction-sets. Please edit the gcc_pkgconfig.mk and remove the -mavx2 -mfma options before compilation if you must run on an unsupported platform.