Make fails if desktop-agnostic cannot be found in standard vapi dir

If libdesktop-agnostic has been installed in a non-standard directory (some dir in ~ for example) then make fails:

Making all in daemon
Makefile:696: warning: overriding commands for target `dockmanager-common.c'
Makefile:678: warning: ignoring old commands for target `dockmanager-common.c'
  VALAC dockmanager-common.c
error: desktop-agnostic-cfg not found in specified Vala API directories or GObject-Introspection GIR directories
Compilation failed: 1 error(s), 0 warning(s)
make[2]: *** [dockmanager_settings_vala.stamp] Error 1
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2

I have attached a patch that fixes it.