Rename configure.in to configure.ac.

GNU Autoconf manual states (section 3.1):

  Previous versions of Autoconf promoted the name configure.in, which
  is somewhat ambiguous (the tool needed to process this file is not
  described by its extension), and introduces a slight confusion with
  config.h.in and so on (for which '.in' means "to be processed by
  configure"). Using configure.ac is now preferred.

autogen.sh

@@ -7,7 +7,7 @@ test -z "$srcdir" && srcdir=.
 PKG_NAME="Evolution"
 REQUIRED_AUTOMAKE_VERSION=1.6
 
-(test -f $srcdir/configure.in \
+(test -f $srcdir/configure.ac \
   && test -f $srcdir/ChangeLog \
   && test -d $srcdir/shell) || {
     echo -n "**Error**: Directory "\`$srcdir\'" does not look like the"