Silence autotools warning

Modern autotools complain that

warning: autoconf input should be named 'configure.ac', not
'configure.in'

The file is explicitly used in autogen.sh, which needs to be updated
consistently.

autogen.sh

@@ -37,7 +37,7 @@ if [ -z "$LIBTOOL" ]; then
   fi
 fi
 
-(grep "^AM_PROG_LIBTOOL" $srcdir/configure.in >/dev/null) && {
+(grep "^AM_PROG_LIBTOOL" $srcdir/configure.ac >/dev/null) && {
   ($LIBTOOL --version) < /dev/null > /dev/null 2>&1 || {
     echo
     echo "**Error**: You must have \`libtool' installed to compile Mono."
@@ -47,8 +47,8 @@ fi
   }
 }
 
-grep "^AM_GNU_GETTEXT" $srcdir/configure.in >/dev/null && {
-  grep "sed.*POTFILES" $srcdir/configure.in >/dev/null || \
+grep "^AM_GNU_GETTEXT" $srcdir/configure.ac >/dev/null && {
+  grep "sed.*POTFILES" $srcdir/configure.ac >/dev/null || \
   (gettext --version) < /dev/null > /dev/null 2>&1 || {
     echo
     echo "**Error**: You must have \`gettext' installed to compile Mono."
@@ -95,7 +95,7 @@ xlc )
 esac
 
 
-if grep "^AM_PROG_LIBTOOL" configure.in >/dev/null; then
+if grep "^AM_PROG_LIBTOOL" configure.ac >/dev/null; then
   if test -z "$NO_LIBTOOLIZE" ; then 
     echo "Running libtoolize..."
     ${LIBTOOL}ize --force --copy
@@ -114,7 +114,7 @@ aclocal $ACLOCAL_FLAGS || {
   exit 1
 }
 
-if grep "^AC_CONFIG_HEADERS" configure.in >/dev/null; then
+if grep "^AC_CONFIG_HEADERS" configure.ac >/dev/null; then
   echo "Running autoheader..."
   autoheader || { echo "**Error**: autoheader failed."; exit 1; }
 fi