Save makefile var clobbering

Makefile

@@ -178,6 +178,10 @@ CPPFLAGS += $(CPPFLAGS_$(CONFIG))
 DEFINES += $(DEFINES_$(CONFIG)) INSTALL_PREFIX=\"$(prefix)\"
 LDFLAGS += $(LDFLAGS_$(CONFIG))
+ifdef EXTRA_DEFINES
+DEFINES += EXTRA_DEFINES
+endif
+
 CFLAGS += -std=c89 -pedantic
 ifeq ($(HAS_CXX11),true)
 CXXFLAGS += -std=c++11

templates/Makefile.template

@@ -195,6 +195,10 @@ CPPFLAGS += $(CPPFLAGS_$(CONFIG))
 DEFINES += $(DEFINES_$(CONFIG)) INSTALL_PREFIX=\"$(prefix)\"
 LDFLAGS += $(LDFLAGS_$(CONFIG))
 
+ifdef EXTRA_DEFINES
+DEFINES += EXTRA_DEFINES
+endif
+
 CFLAGS += -std=c89 -pedantic
 ifeq ($(HAS_CXX11),true)
 CXXFLAGS += -std=c++11

tools/run_tests/run_tests.py

@@ -201,7 +201,7 @@ make_targets = []
 languages = set(_LANGUAGES[l] for l in args.language)
 build_steps = [jobset.JobSpec(['make',
                                '-j', '%d' % (multiprocessing.cpu_count() + 1),
-                               'DEFINES=GRPC_TEST_SLOWDOWN_MACHINE_FACTOR=%f' % args.slowdown,
+                               'EXTRA_DEFINES=GRPC_TEST_SLOWDOWN_MACHINE_FACTOR=%f' % args.slowdown,
                                'CONFIG=%s' % cfg] + list(set(
                                    itertools.chain.from_iterable(
                                        l.make_targets() for l in languages))))