[PATCH] Configure: call "make distclean" for zlib only if Makefile exists

[Piotr Sikora]

Hey,

> Configure: call "make distclean" for zlib only if Makefile exists.
>
> This change allows to build nginx against git checkout of zlib.

Actually, it looks that only our amd64-optimized version [1] doesn't
ship with Makefile, the original repository [2] has a minimal one with
"distclean" target, so feel free to ignore this patch (although, I
still think it's a good practice to check if the Makefile exists
before calling make).

[1] https://github.com/cloudflare/zlib/commits/gcc.amd64
[2] https://github.com/madler/zlib

Best regards,
Piotr Sikora