ax_gcc_archflag - Support for Skylake and newer Intel platforms?

Dear autoconf devs.,

Would it be possible to update the AX_GCC_ARCHFLAG() macro to correctly 
detect the Intel Skylake, Kaby Lake and Coffee Lake CPUs ?
As it stands the macro defaults to '-march=core2', which misses the 
AVX2, FMA and other extensions.

Thanks for your help!

Matthieu