liblwgeom: new formula to build just liblwgeom from postgis

Does not require postgresql (if not building HEAD) or gdal.

Can be used to address https://github.com/mxcl/homebrew/issues/20373 and finally bring lwgeom support back to libspatialite.

Also, this lib is being considered for inclusion in QGIS, to bring much-needed, on-the-fly geometry repairing. So, it will probably be part of a new qgis.rb for version 2.0/HEAD.

Caveat: relies on a patch to strip all the PostgreSQL functions from PostGIS configure.ac. Not ideal, but upstream developers for PostGIS have mentioned they do not intend to separate the build process for lwgeom out from PostGIS.