Revert "Build: Don't assume that the xmlpatterns command is available."

The check does not work on Windows since it does not take
the .exe suffix into account.

Task-number: QTCREATORBUG-11768

This reverts commit dcd71423d25baab4c797098a0363c51ea164e99f.

Change-Id: I448a449b7749ee533c43a5b8007eedca7380cff8
Reviewed-by: Eike Ziller <eike.ziller@digia.com>