Adds --with-pcre=<install-dir/pcre-expanded-dir> to Nginx configure flags

Only included when installing on OSX. Otherwise build fails 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.
Merge pull request #1212 from sjcorbett/fix/ngingx-install-osx

Fix Nginx install on OSX