Fix Nginx install on OSX

Adds `--with-pcre=/path/to/expanded/pcre/dir` to flags for Nginx `./configure` when `isMac` is true.

Previously build failed with:
>./configure: error: the HTTP rewrite module requires the PCRE library.
You can either disable the module by using --without-http_rewrite_module
option, or install the PCRE library into the system, or build the PCRE
library statically from the source with nginx by using --with-pcre=<path> option.