autopoint: When both configure.ac and configure.in exist, use the former.
Reported by Eric Gallager <egall@gwmail.gwu.edu> in
<https://lists.gnu.org/archive/html/bug-gettext/2020-12/msg00051.html>.
* gettext-tools/misc/autopoint.in (configure_in): Test for configure.ac before
testing for configure.in.
