Cross compiling Heimdal

We got some feedback that it would be good if it was possible to cross compile Heimdal and with some minor works that is now possible.

Its all documented at http://www.h5l.org/compile.html#cross, as usual libtool is somewhat in the way. The current problem that that libtool is not aware of the target's build environment, but it seems to work anyway. Oh well.

The code is all patch of master and will be in the soon to be release Heimdal 1.3.