Fixes for build on centos 7/8, SIMDE, auto-detect AVX2/512 and architecture

The branch includes fixes to allow build on centos 7 and centos 8 (centos stream 8) hosts. In addition it fixes the support of a non-avx2 host using simde (eth_udp.c was not using the simde_mm256 calls but native calls instead and the compile flags were not set properly for a non-avx2 host, preventing from successful build on such a host). Cmake autodetects whether AVX2/512 is present and sets the right flag, as well as the CPU architecture.