[IMPROVE] adding support for custom ZLIB and PCRE build

[mtx-z]

Hello,

awesome job with this script.
I wondered if you would be interested in a PR to be able to build Nginx with custom/latest PCRE and Zlib libs?

--with-zlib=
--with-pcre=

I usually set those in my configure option. It's almost the same process as for openSSL: git clone, configure, make.

I just want to know if it's useful or not, as maybe there's a good reason those are not specified.

Thanks

EDIT: related to #176 - may be nice to be able to specify official zlib version, or custom (Cloudflare) one.