Fix configuration for cross-compilation

This is a fix to https://bugzilla.samba.org/show_bug.cgi?id=13846, which does not involve adding private patches to waf. Instead, the fix upgrades waf from 2.0.17 to 2.0.18 (from one pristine copy to another, nothing to port to the newer version).

Commit #4 works around what I believe to be a bug in rpath detection, but there doesn't seem to be a bug in rpath build. I suppose I can demonstrate that by a native build on a Linux machine with rpath support inhibited in the dynamic linker (if such a setup would not break the system completely).

Commit #5 adds some tests, so as to make catching a degradation more probable in the future. I didn't use knownfail because this is not part of the selftest suite, and we don't seem to have a knownfail at the autobuild level.