autobuild: Add test for cross-compilation infrastructure

autobuild: Add test for cross-compilation infrastructure

With added option to generate cross-answers by cross-execute script,
a test is added to verify that at least for tested build environments,
the infrastructure is self-consistent.

The test runs samba configuration in three modes:
1. Native
2. Cross compile with cross-execute, using the build machine as its own
   target
3. Cross compile with cross-answers, using the answers generated in
   step 2

The build environments created by the three processes are then tested to
be equal.

Signed-off-by: Uri Simchoni <urisimchoni@gmail.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Wed May 20 14:10:05 CEST 2015 on sn-devel-104