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/
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