build/run against zlib < 1.2.3.5 results in panic in uncompress_stdmac()

[sezero]

build/run against zlib < 1.2.3.5 results in panic in uncompress_stdmac():
https://github.com/netwide-assembler/nasm/blob/master/asm/uncompress.c#L43-L44

0 as windowBits is illegal in inflateInit2() from zlib < 1.2.3.5 :
https://github.com/madler/zlib/blob/develop/ChangeLog#L613

Ran into this on (yes: outdated/eol) CentOS 6.

It would be nice that nasm detect old zlib at configure time and fall
back to embedded version.

[hpax]

This is trivially fixed by changing 0 to 15 in asm/uncompress.c.