nginx: Makes the build script to check for deps

The ModSecurity module has some dependencies that may not be part
of the Nginx build, and so it is not part of the configuration
options. In order to resolve this dependencies automatically this
`config' is now making use of the pkg-config, utility that is
popular to make this compatible layer between different Linux
distributions and operational systems.
Merge pull request #147 from zimmerle/fix_nginx_build_config_top_of_trunk

Try 3 - nginx: Makes the build script to check for deps