Add make variable to allow testing of different interpreter flags in buildbot runs

Makefile.pre.in

@@ -740,7 +740,7 @@ $(LIBRARY_OBJS) $(MODOBJS) Modules/python.o: $(PYTHON_HEADERS)
 
 TESTOPTS=	-l $(EXTRATESTOPTS)
 TESTPROG=	$(srcdir)/Lib/test/regrtest.py
-TESTPYTHON=	$(RUNSHARED) ./$(BUILDPYTHON) -Wd -E -bb
+TESTPYTHON=	$(RUNSHARED) ./$(BUILDPYTHON) -Wd -E -bb $(TESTPYTHONOPTS)
 test:		all platform
 		-find $(srcdir)/Lib -name '*.py[co]' -print | xargs rm -f
 		-$(TESTPYTHON) $(TESTPROG) $(TESTOPTS)