[SAMBA4][PATCH] allow srcdir and builddir to differ in the build system

[tridge at samba.org]

James,

thanks, I would definately like to get builds out of the source tree
working! I tried your patch, and it worked well for me.

Jelmer and Metze, are you happy with the approach James has taken?
If so, we should get this in then enable out of tree builds for Samba4
in the build farm (so we will know when/if it breaks)

Cheers, Tridge