imageburner: Calculate gmock/gtest library dependencies programatically

(gmock/gtest doesn't generate pkg-config metadata, calculating lib
 dependencies this way makes it easier to upgrade gmock/gtest packages.)

BUG=chromium:211445
TEST=build and run tests for package using gtest 1.6

Change-Id: I11e552297e5d5b99ee8387b0388b1f51ffb0d616
Reviewed-on: https://gerrit.chromium.org/gerrit/46421
Reviewed-by: <tbarzic@google.com>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Tested-by: Gaurav Shah <gauravsh@chromium.org>
Commit-Queue: Gaurav Shah <gauravsh@chromium.org>
diff --git a/Makefile b/Makefile
index e506574..4a03e4c 100644
--- a/Makefile
+++ b/Makefile
@@ -37,8 +37,9 @@
 clean:
 	rm -rf *.o $(BINDINGS_DIR)/* marshal.h $(BINS)
 
+UNITTEST_LIBS := $(shell gmock-config --libs) $(shell gtest-config --libs)
 unittest_runner: unittest_runner.o image_burner_impl_unittest.o $(COMMON_OBJS)
-	$(CXX) $(CXXFLAGS) $(LDFLAGS) $^ $(LDLIBS) -lgmock -lgtest -o $@
+	$(CXX) $(CXXFLAGS) $(LDFLAGS) $^ $(LDLIBS) $(UNITTEST_LIBS) -o $@
 
 image_burner_tester.o: bindings/client.h
 image_burner_tester: image_burner_tester.o