Use += not = to set makefile variables after including base makefiles.

Use += not = to set makefile variables after including base makefiles.

The previous coding in hstore_plpython and ltree_plpython wiped out any
values set by the base makefiles.  This at least had the effect of running
the tests in "regression" not "contrib_regression" as expected.  These
being pretty new modules, there might be other bad effects we'd not
noticed yet.