Add AM_MAINTAINER_MODE([enable]) to spatialindex' configure.ac

This adds the options --enable/--disable-maintainer-mode to configure.
With --disable spatialindex' Makefiles will no longer try to reconfigure/
autoreconf/automake when it thinks intermediate files (like Makefile.in
and configure) are out of date. This prevents random regeneration of these
when timestamps are messed up by switching branches in git.

This commit probably undoes the following two previous commits:

skramer@skramer-laptop:~/git/fluidity$ git log -n2 spatialindex-1.8.0
commit 51948d2533b43952f8916c58df2f6ca71324b8cd
Author: Gerard Gorman <g.gorman@imperial.ac.uk>
Date:   Thu Jan 31 17:43:38 2013 +0000

    Removed spaces from PACAKGE_STRING in spatialindex as it causes the intel compiler to choke.

commit 8a8fe4cad7dd9ac29c6f6208b17fbbbc466c8421
Author: Gerard Gorman <g.gorman@imperial.ac.uk>
Date:   Tue Jan 29 11:07:31 2013 +0000

    Stop spatialindex from executing make test; this fixes compilation on Cray and other systems where we compile statically.

If we hit these issues again I'd rather look for a proper fix instead
of creating an inconsistent set of build files (these were fixes
were done in configure instead of configure.ac).