verity: 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 tests for package using gtest 1.6

Change-Id: I746d9d422a18ee2f3feeeeade9e85f267739eb35
Reviewed-on: https://gerrit.chromium.org/gerrit/46446
Reviewed-by: Will Drewry <wad@chromium.org>
Tested-by: Gaurav Shah <gauravsh@chromium.org>
Commit-Queue: Gaurav Shah <gauravsh@chromium.org>
diff --git a/Makefile b/Makefile
index bffde42..b3ab634 100644
--- a/Makefile
+++ b/Makefile
@@ -57,7 +57,8 @@
                           $(filter %dm-bht-userspace.o,$(C_OBJECTS)) \
                            CXX_STATIC_LIBRARY(kernel/libkernel.pie.a) \
                            CXX_STATIC_LIBRARY(simple_file/libsimple_file.pie.a)
-CXX_BINARY(verity_testrunner): LDLIBS += -lgtest -lgmock -lpthread
+UNITTEST_LIBS := $(shell gmock-config --libs) $(shell gtest-config --libs)
+CXX_BINARY(verity_testrunner): LDLIBS += $(UNITTEST_LIBS)
 
 all: CXX_BINARY(verity_testrunner)
 clean: CLEAN(verity_testrunner)