Fix out-of-tree builds.

XXX file upstream bug

--- librsvg-2.40.9/Makefile.am 2015-02-04 14:17:14.000000000 -0800

+++ librsvg-2.40.9/Makefile.am 2015-05-22 16:26:28.059926082 -0700

@@ -254,7 +254,7 @@

librsvg_@RSVG_API_VERSION_U@_vapi_DEPS = gio-2.0 cairo

librsvg_@RSVG_API_VERSION_U@_vapi_METADATADIRS = $(srcdir)

-librsvg_@RSVG_API_VERSION_U@_vapi_FILES = Rsvg-$(RSVG_API_VERSION).gir Rsvg-$(RSVG_API_VERSION)-custom.vala

+librsvg_@RSVG_API_VERSION_U@_vapi_FILES = Rsvg-$(RSVG_API_VERSION).gir $(srcdir)/Rsvg-$(RSVG_API_VERSION)-custom.vala

vapidir = $(datadir)/vala/vapi

vapi_DATA = $(VAPIGEN_VAPIS)