The install command on MacOS does not have the -t command line flag, so it fails.

Additionally, the folder name for the libraries generated was not "lib" but "libmpc.so.dSYM".

I installed the files manually by copying them and then manually resetting their permissions afterward:

cp -r ./$(DIST)/libmpc.so.dSYM $(DESTDIR)$(PREFIX)/lib
cp ./$(PROJ).c $(DESTDIR)$(PREFIX)/share/$(PROJ)
cp ./$(PROJ).h $(DESTDIR)$(PREFIX)/share/$(PROJ)