portable make fix

git-svn-id: svn://bind10.isc.org/svn/bind10/branches/trac315@2774 e5f2f494-b856-4b98-b285-d166d9295462

src/lib/config/testdata/Makefile.am

@@ -5,4 +5,4 @@ BUILT_SOURCES = b10-config.db
 # cfgmgr_test (under lib/python) will override b10-config.db, so we make a
 # writable copy in the builddir.
 b10-config.db: b10-config.db.master
-	cp $< $@
+	cp $(srcdir)/b10-config.db.master $@