Fix up dependency finding for libg{mock,test}_main

- Look up the corresponding non-*_main libraries via LDFLAGS using the
  directories provided in src.libnames.mk. This will allow the libraries to
  be built in the "make libraries" phase of buildworld.
- gtest_main relies on gtest, but didn't explicitly call out the dependency
  in `LIBADD`. Fill in this missing blank.

lib/googletest/gmock_main/Makefile

@@ -12,7 +12,11 @@ CXXFLAGS+=	-I${GOOGLEMOCK_SRCROOT}/include
 CXXFLAGS+=	-I${GOOGLEMOCK_SRCROOT}
 CXXFLAGS+=	-I${GOOGLETEST_SRCROOT}/include
 
-LIBADD=		gmock
+LIBADD+=	gmock
+# NB: LDFLAGS added for buildworld builds
+LDFLAGS+=	-L${LIBGTESTDIR}
+LDFLAGS+=	-L${LIBGMOCKDIR}
+
 SRCS+=		gmock_main.cc
 
 SUBDIR.${MK_TESTS}+=	tests

lib/googletest/gtest_main/Makefile

@@ -11,6 +11,10 @@ PRIVATELIB=	true
 CXXFLAGS+=	-I${GOOGLETEST_SRCROOT}/include
 CXXFLAGS+=	-I${GOOGLETEST_SRCROOT}
 
+LIBADD+=	gtest
+# NB: LDFLAGS added for buildworld builds
+LDFLAGS+=	-L${LIBGTESTDIR}
+
 SRCS+=		gtest_main.cc
 
 SUBDIR.${MK_TESTS}+=	tests