Fix a timestamp race in python-missing.

Without this, it's possible for mypy.m4 and aclocal.m4 to end up with the same
timestamp as configure, so autoconf doesn't bother rebuilding it, and it just
reruns the previous AM_PATH_PYTHON test, succeeding rather than failing.

diff -x config.log -x config.status -ru tmp/automake-1.12.3/t/python-missing.sh work/automake-1.12.3/t/python-missing.sh
--- tmp/automake-1.12.3/t/python-missing.sh	2012-08-13 17:40:13.000000000 +0100
+++ work/automake-1.12.3/t/python-missing.sh	2012-08-16 15:35:05.569005088 +0100
@@ -56,6 +56,9 @@
 
 rm -rf autom4te*.cache # Will have to re-run aclocal.
 
+# Make sure mypy.m4's timestamp becomes newer than configure's.
+$sleep
+
 # Hopefully the Python team will never release such a version :-)
 echo 'AM_PATH_PYTHON([9999.9], [])' > mypy.m4
 $ACLOCAL