build: Use @libdir@ in the .pc.in for the value of libdir

This solves problems on systems where the hardcoded value of
${exec_prefix}/lib is not correct. Fixes issue #21.

dbus-sharp-glib-1.0.pc.in

@@ -1,6 +1,6 @@
 prefix=@prefix@
 exec_prefix=${prefix}
-libdir=${exec_prefix}/lib
+libdir=@libdir@
 
 Name: Managed DBus GLib integration
 Description: GLib integration for dbus-sharp, the D-Bus IPC library