mapnik: Harden build. New: --with-gdal --with-geos

- Mapnik searches for py2cairo -> added dep if --with-cairo
- Explicitly give includes and libs for icu, boost, proj,
  jpeg, libtif, and cairo
- pkg-config is not only used for cairo -> moved it out
  of the `if build.include? 'with-cairo'` block

Mapnik previously failed on superenv, if homebrew is in a non-standard location. 
With these changes, I can build it successfully on Xcode-only 10.8.