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.

contrib/hstore_plpython/Makefile

@@ -28,10 +28,10 @@ ifeq ($(PORTNAME), win32)
 SHLIB_LINK += ../hstore/libhstore.a $(wildcard ../../src/pl/plpython/libpython*.a) $(wildcard ../../src/pl/plpython/libplpython*.a)
 endif
 
-REGRESS_OPTS = --load-extension=hstore
+REGRESS_OPTS += --load-extension=hstore
 ifeq ($(python_majorversion),2)
 REGRESS_OPTS += --load-extension=plpythonu --load-extension=hstore_plpythonu
 endif
-EXTRA_INSTALL = contrib/hstore
+EXTRA_INSTALL += contrib/hstore
 
 include $(top_srcdir)/src/pl/plpython/regress-python3-mangle.mk

contrib/ltree_plpython/Makefile

@@ -28,10 +28,10 @@ ifeq ($(PORTNAME), win32)
 SHLIB_LINK += $(wildcard ../../src/pl/plpython/libpython*.a)
 endif
 
-REGRESS_OPTS = --load-extension=ltree
+REGRESS_OPTS += --load-extension=ltree
 ifeq ($(python_majorversion),2)
 REGRESS_OPTS += --load-extension=plpythonu --load-extension=ltree_plpythonu
 endif
-EXTRA_INSTALL = contrib/ltree
+EXTRA_INSTALL += contrib/ltree
 
 include $(top_srcdir)/src/pl/plpython/regress-python3-mangle.mk