Fix generated pkg-config file

dune-tectonic.pc.in

@@ -11,5 +11,5 @@ Version: @VERSION@
 Description: dune-tectonic module
 URL: http://dune-project.org/
 Requires: dune-common dune-fufem dune-tnnmg
-Libs: -L
-Cflags: -I
+#Libs: -L${libdir} -ldunetectonic
+Cflags: -I${includedir}