The mingw-w64-tool package in Fedora provides widl tool with a
cross prefix, so adds it automatically for cross builds.

WIDL env can be used to redefine the path to tool.
The same behavior as with windres.

Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Message-Id: <20220428181525.300521-1-kkostiuk@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>