Use correct gtest/gmock flags

The test Makefile.am was using -lgmock -lgmock_main directly, which
cause link issues on compiling test.
Fix this by using the flags specified by gtest package.

Tested: Verify the CI passes build.

Change-Id: Id51134d7e2696e18f7439f15f8d899edba51856e
Signed-off-by: Lei YU <mine260309@gmail.com>
diff --git a/test/Makefile.am b/test/Makefile.am
index 3071c72..0c14421 100644
--- a/test/Makefile.am
+++ b/test/Makefile.am
@@ -15,14 +15,16 @@
 test_LDADD = $(top_builddir)/libtimemanager.la
 
 test_CPPFLAGS = $(GTEST_CPPFLAGS) \
+                $(GMOCK_CFLAGS) \
+                $(GTEST_MAIN_CFLAGS) \
                 $(AM_CPPFLAGS)
 
 test_CXXFLAGS = $(PTHREAD_CFLAGS) \
                 $(PHOSPHOR_DBUS_INTERFACES_CFLAGS) \
                 $(SDBUSPLUS_CFLAGS)
 
-test_LDFLAGS = -lgmock_main \
-               -lgmock \
+test_LDFLAGS = $(GMOCK_LIBS) \
+               $(GTEST_MAIN_LIBS) \
                $(PTHREAD_LIBS) \
                $(OESDK_TESTCASE_FLAGS)