Fix parallel postgresql build on macOS

[deciduously]

When using the default parallel setting, an installation step appears to run twice and clobber itself:

rm -f libpgtypes.dylib
rm -f libpgtypes.dylib
rm -f libpq.dylib
ln -s libpgtypes.3.dylib libpgtypes.dylib
ln -s libpgtypes.3.dylib libpgtypes.dylib
ln -s libpq.5.dylib libpq.dylib
make[5]: Leaving directory '/Users/benlovy/.tangram/tmp/06cnw782j1xs703yke4djsev88/work/src/interfaces/ecpg/pgty
peslib'
fil_01ab9gqxdnky43xhj4wrstza8ffy9v7v44r01je1y5bbj29bc0xwp0: failed to create symbolic link 'libpgtypes.dylib': F
ile exists
make[6]: Leaving directory '/Users/benlovy/.tangram/tmp/06cnw782j1xs703yke4djsev88/work/src/interfaces/libpq'
make[6]: *** [/Users/benlovy/.tangram/artifacts/dir_01etdcw4gm7mtjvrj6jy72rreszafrtppy6hvbkyz4dyfft6cggaqg/src/M
akefile.shlib:263: libpgtypes.3.dylib] Error 1
make[6]: Leaving directory '/Users/benlovy/.tangram/tmp/06cnw782j1xs703yke4djsev88/work/src/interfaces/ecpg/pgty
peslib'
make[5]: *** [Makefile:50: submake-pgtypeslib] Error 2
make[5]: Leaving directory '/Users/benlovy/.tangram/tmp/06cnw782j1xs703yke4djsev88/work/src/interfaces/ecpg/ecpg
lib'
make[4]: *** [Makefile:42: submake-ecpglib] Error 2
make[4]: Leaving directory '/Users/benlovy/.tangram/tmp/06cnw782j1xs703yke4djsev88/work/src/interfaces/ecpg/comp
atlib'
make[3]: *** [Makefile:17: all-compatlib-recurse] Error 2
make[3]: *** Waiting for unfinished jobs....
touch preproc.h

For now, the build is done without the -j setting which alleviates the issue, but slows down the build.