Added AVX2 build option for gcc. The -march=core-avx2 and -mavx2 were added if ENABLE_AVX2 option is chosen at the time of configure.
A debug flag -DENABLE_AVX2 was also added as a preliminary step to enable AVX2 instructions where applicable such as memset, memchr

It's better to explicitly choose AVX2 build option since the BUILD and TARGET systems may differ. Auto detecting this feature from CPUID would fail if the TARGET system does not support AVX2 instructions.