Several fixes for configure script

[jeroen]

Fixes #87. This fixes a few things:

• Fix the build for cross-compilation, by using the pkg-config version instead of a test program.
• Adapt the cut command for when proj version has 2 digits (v10) such that we can't just grab the first character
• Actually set POSTGIS_PROJ_VERSION from the configure script instead of hardcoding it to 91.

See also in sf: r-spatial/sf#2270

[edzer]

It seems like packages sf and terra, who build also require GEOS, skip the entire ./configure step, and compile, while lwgeom breaks on a geos test in configure. How can we skip this for wasm / em++?