pgsql: Use += not = to set makefile variables after including base make

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.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/b14cf229f4bd7238be2e31d873dc5dd241d3871e

Modified Files
--------------
contrib/hstore_plpython/Makefile |    4 ++--
contrib/ltree_plpython/Makefile  |    4 ++--
2 files changed, 4 insertions(+), 4 deletions(-)