cmake: Fix validation of waffle_has_PLATFORM options
cmake: Fix validation of waffle_has_PLATFORM options

On Linux, at least one of waffle_has_{glx,wayland,x11_egl} must be chosen.
The user can opt out of all X11 platforms and choose only Wayland.

Signed-off-by: Chad Versace <a557f7f75bf28dc19ac99107f5cc7cd6b3fbdb1a@linux.intel.com>

waffle-gl/waffle,evelikov/waffle,fjhenigman/waffle,fjhenigman/waffle,dcbaker/waffle,fjhenigman/waffle,janesma/waffle,chadversary/waffle,evelikov/waffle,janesma/waffle,dcbaker/waffle,maurossi/waffle,chadversary/waffle,janesma/waffle,yinquan529/waffle-gl-waffle,yinquan529/waffle-gl-waffle,chadversary/waffle,waffle-gl/waffle,maurossi/waffle,waffle-gl/waffle,maurossi/waffle,waffle-gl/waffle,evelikov/waffle,yinquan529/waffle-gl-waffle,dcbaker/waffle