Check submodules before running autoconf.

Exit early with an informative message if the submodules are missing,
since they are needed.  Without this autoconf throws a bunch of
uninformative errors which does not point to the actual problem.
This was taken from util-keysyms.

Signed-off-by: Niclas Zeising <zeising@daemonic.se>
Reviewed-By: Arnaud Fontaine <arnau@debian.org>