Build fixes

Some fixes for build environments where dependencies are installed in
distinct directories.

Change-Id: I38808fd2911747b266ee6fde91187a88dd7ae355
diff --git a/openbsc/tests/xid/Makefile.am b/openbsc/tests/xid/Makefile.am
index e7c4cf0..aaf17ed 100644
--- a/openbsc/tests/xid/Makefile.am
+++ b/openbsc/tests/xid/Makefile.am
@@ -32,7 +32,7 @@
 	$(LIBOSMOGB_LIBS) \
 	$(LIBCARES_LIBS) \
 	$(LIBCRYPTO_LIBS) \
-	-lgtp \
+	$(LIBGTP_LIBS) \
 	-lrt \
 	-lm \
 	$(NULL)