libxcvt: allow native build

libxcvt: allow native build

'mutter' requires the 'cvt' binary at compile time to build the native backend
For this it depends on xserver-xorg-cvt-native, which is currently broken and
also deprecated.
[https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/637]

This commit would allow to use libxcvt-native to provide the needed binary
instead.

Signed-off-by: Markus Volk <f_l_k@t-online.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>