macos: use GAUDI_LIBRARY_PATH instead of LD_...

This is a quick, macOS-only implementation of the suggestion in #251 (closed) , finally fixing Gaudi on macOS for me (after setting export GAUDI_LIBRARY_PATH=$LD_LIBRARY_PATH)

Should be safe to merge as the changes are only inside APPLE ifdefs, but does make the code a bit more complicated.