Since the platform is selected at run time (e.g. xcb vs. Wayland), it
is not a good idea to decide at compile time what code is run. Another
example is on macOS, where having xcb libraries available will lead to
lauching the Xquartz whenever LyX is running.
With this patch, things are separated:
- the code is run when run-time platform is "xcb".
- the support code is compiled in when xcb header and libraries are
available.
The cmake code is courtesy of Kornel Benko.
Fixes ticket #13086.
(cherry picked from commit 18c310a8cf)
(cherry picked from commit f1fddd86ef)
(cherry picked from commit 3bc4554e57)
(cherry picked from commit 8c14748e78)
d7e2ca377f