Use $(PKG_CONFIG), and also avoid repeating the calls for every objects.
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -2,2 +2,2 @@
-CXXFLAGS+=-Wall -std=c++14 `pkg-config --cflags gtest_main`
-LDFLAGS+=`pkg-config --libs gtest_main`
+CXXFLAGS:=-Wall -std=c++14 $(shell $(PKG_CONFIG) --cflags gtest_main) $(CXXFLAGS)
+LDFLAGS:=$(LDFLAGS) $(shell $(PKG_CONFIG) --libs gtest_main)