Rename configure.in to configure.ac
The old name was deprecated in 2013.

See-also: https://lists.gnu.org/archive/html/automake/2013-05/msg00049.html
Suggested-by: lintian, aclocal
X: libpst source: deprecated-configure-filename
aclocal: warning: autoconf input should be named 'configure.ac', not 'configure.in'