Created a CMakeLists.txt file for the x11new port

[frostbane]

This is a rough translation of the x11new Makefile.

[GitMensch]

Can you please add that to the main CMakeLists.txt file with an optional subdir build?
Ideally also adding the build cmake in the GH action, but that would be optional.

[frostbane]

I added it to the main CMakeLists.txt but I didn't make use of the project_common.cmake.
I added PDC_X11_BUILD option to add the subdirectory optionally (defaults to OFF).

I never used GH actions, so maybe keep it tbd...?

[GitMensch]

There's no Appveyor run of x11 at all and GH actions only test vt11.
"Looks good in general" and as it seem to work for you as well I'm going to squash-merge as is.