bug#63252: Graphviz uses bundled copy of libltdl

["Ludovic Courtès" <ludo@gnu.org>]

As of 44d70440944aa47e5f37493346e560f38907d777, Graphviz uses a bundled
copy of libltdl (GNU Libtool):

--8<---------------cut here---------------start------------->8---
make[3]: Entering directory '/tmp/guix-build-graphviz-7.0.1.drv-0/graphviz-7.0.1/libltdl'
  CC       libltdlc_la-lt__alloc.lo
  CC       libltdlc_la-lt_dlloader.lo
  CC       libltdlc_la-ltdl.lo
  CC       libltdlc_la-lt_error.lo
  CC       libltdlc_la-slist.lo
  CC       lt__strl.lo
  CC       loaders/dlopen.lo
  CC       loaders/libltdlc_la-preopen.lo
  CCLD     dlopen.la
ar: `u' modifier ignored since `D' is the default (see `U')
  CCLD     libltdlc.la
ar: `u' modifier ignored since `D' is the default (see `U')
make[3]: Leaving directory '/tmp/guix-build-graphviz-7.0.1.drv-0/graphviz-7.0.1/libltdl'
--8<---------------cut here---------------end--------------->8---

We should make sure it uses our own copy instead.

Ludo’.