Autotools update

Could we have an updated version of ax_ext.m4 such that AVX2 and more recent versions can be detected ? As a consequence, could we set the correct flags for the Intel compiler ? -mAVX (for instance) is not the best choice for AVX. -xAVX is better. @jwillis should have a list of flags to use in the case where AVX is available and when AVX2 is available.

Also, could we have the inter-procedural optimissation flags (-ipo for Intel) as an option somewhere ? It seems to give a slight improvement to the performance. Note that when doing this we might have to use xild instead of ld and xiar instead of ar.