Use INSTALL_NAME_DIR for MacOS

[pramsey]

Due to some apparent changes in MacOS, we need to start generating dylibs that either use the explicit install location in their path information, which would require using the INSTALL_NAME_DIR directive, or start including LC_RPATH entries in the libraries, which might involve using CMAKE_INSTALL_RPATH on the library targets.

I think the main benefit of using the latter is that it can be re-written after build, which since we test our libraries before install time might be required.

Otherwise we might find that after building geos_test_unit against our local copies it nonetheless loads the system copy from /usr/local/lib when we run it.

[pramsey]

I'm going to close this for now, as I have instead updated the PostGIS build to set the rpath for GEOS and Proj and GDAL as necessary.

[dbaston]

Paul's fix for the PostGIS build: postgis/postgis@14464b4