Upstream STP's libstp now depends on boost.

This commit updates the configure script to detect this by first trying to link without
boost and if that fails then trying to link libstp with boost.

This also updates the relevant Makefiles so that the klee and kleaver
executables link in STP's boost dependencies if necessary.

@hpalikareva I've not tested how this interacts with the Meta-SMT build of KLEE