wafsamba: avoid pre-forking if cross-compilation is enabled

wafsamba: avoid pre-forking if cross-compilation is enabled

Waf supports pre-forking to run configuration tests, but this
doesn't play well with Samba's cross-compilation support, because
Samba monkey-patches the actual fork+exec, which doesn't happen
in a pre-forked process pool.

This patch emulates the impact of WAF_NO_PREFORK env var when
cross-compilation is enabled.

The blueprint for the solution has been suggested by Thomas Nagy
in https://bugzilla.samba.org/show_bug.cgi?id=13846#c7 (item #2)

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13846

Signed-off-by: Uri Simchoni <uri@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>