Option for static linking of proj4 in gdal

When trying to build gdal in a non-`/usr/local/` location, unfortunately it cannot find `libproj.dylib` during runtime use of the gdal library, and I cannot see any compile-time configuration option that allows one to point to the custom path (unlike for basically everything else, e.g. options like `--with-geotiff=`).

One compile-time option that gets around this, though, is statically linking proj4 into gdal.  So this is an addition to the gdal formula adding that option.

I'm guessing that ideally one could just give the path to libproj.dylib, but in this one case I don't see anything other than runtime options (`PROJSO`) allowing for that path to be set.