Fix the build on GCC 4.x after 812e8f2a1
Fix the build on GCC 4.x after 812e8f2a1

The ancient GCC 4.x doesn't understand the "optimize" attribute until 4.4.
Fix the build on platforms with GCC 4.x < 4.4 by limiting the DONT_VECTORIZE
definition to GCC 5 and greater.

Noticed and patch proposed by Warner Losh <imp@FreeBSD.org>.

Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd