Make update-symbol-files work when builddir != srcdir
Make update-symbol-files work when builddir != srcdir

rewrite the update-symbol-files rule to use dependencies and automatic
variables ($^) so that VPATH will allow make to find these files in the
srcdir as well as the builddir.

freedesktop-unofficial-mirror/spice__spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,Fantu/spice-gtk,flexVDI/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,Fantu/spice-gtk,fgouget/spice-gtk,Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,flexVDI/spice-gtk,Fantu/spice-gtk