Minor Makefile fixes and tidying.
Squashed commit of the following:

commit 2397f19ea6086d194b083fa5fdde2920dbb343da
Author: 9thbit <9thbit@gmail.com>
Date:   Sun Jun 9 21:56:44 2013 +0100

    Some Makefile tidying.

commit 908a2cbb10ddd87f1250a6fd279ab2c14f55afd2
Author: 9thbit <9thbit@gmail.com>
Date:   Sun Jun 9 21:41:03 2013 +0100

    Fix some compiler warnings about sign/unsigned comparisons.

commit b2c27b97ad5b58a251fb546a4f99a45433de7faa
Author: 9thbit <9thbit@gmail.com>
Date:   Sun Jun 9 20:45:31 2013 +0100

    Ensure that the SAT and MIP wrappers are compiled before the solver interfaces.