Fix a bug where autoconf 2.71 complains about AC_PROG_INTLTOOL not being used.

Testing Done:
Tested in a fedora rawhide container with the autoconf 2.71 copr installed.
Also tested in debian buster with autoconf 2.71 installed from experimental.

Bugs closed: PIDGIN-17509

Reviewed at https://reviews.imfreedom.org/r/589/