extract_texi_filenames fail for contributor.xref-map out-of-tree build

[Graham Percival]

Doing an out-of-tree build:
  mkdir ../lily-out
  make distclean
  cd ../lily-out
  ../lilypond/configure
  make
  make doc

produces a 0-byte contributor.xref-map and
AAA-intro-reression.xref-map.  It looks fine for INTALL, internals,
snippets, usage, and web.  (I'm surprised that it worked for web and
not contributor)

I'm still building in order to find out if anything else breaks, but
so far I suspect that this breakage is causing GUB to fail.  Recall
that building contributor doesn't produce any .texi files in out-www/.

Cheers,
- Graham