CentOS (and probably other "enterprise" distros) have too old nasm

90e19434 changed from yasm to nasm. x264 needs nasm 2.13.0 at least. CentOS 7.6.1810 has nasm 2.10.07.

So we need to still build nasm ourselves on these old distros, just like we did for yasm before.

@nirbheek