package/speechd: fix libltdl enable/disable configure option

- libltdl usage is configured by --disable/enable-ltdl (see configure.ac
  'AC_ARG_ENABLE([ltdl], [...])')

Fixes:

  configure: WARNING: unrecognized options: [...] --with-libltdl

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>