This change adds a check whether compiler supports some SIMD
(Single Instruction, Multiple Data) instructions. It is enabled
by default, and can be disabled by configure option --disable-simd.
The check macro is based on the AX_EXT from autoconf-archive:
www.gnu.org/software/autoconf-archive/ax_ext.html
And depends on the ax_check_compile_flag macro:
www.gnu.org/software/autoconf-archive/ax_check_compile_flag.html
Currently only the following SIMD extensions are being checked:
AVX2, SSE3, SSE4.1, but adding others is also possible. All found
extensions are being defined in the 'config.h' header.
Change-Id: Idf8fff984bd936a75c7c307338df88ba4b005817
Vadim Yanitskiy