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

[tridge at samba.org]

James,

I spoke a bit too soon. The out of tree build works, but 'make test'
doesn't work, plus we do need to fix the header part of 'make
install'. Both would need to work for us to use this in the build
farm.

Cheers, Tridge