CTABLES is now available

[Ben Pfaff]

I just added support for CTABLES to PSPP. It is already available
through the autobuilder at https://benpfaff.org/~blp/pspp-master/,
including a flatpak build. I guess that the autobuilder at
https://caeis.etech.fh-augsburg.de/buildbot/#/builders will also
have a build soon.

Most CTABLES features are available and I have tested them.
The following features are not implemented:
- Multiple response sets
- SIGTEST
- COMPARETEST
- Summary functions:
  * .LCL and .UCL suffixes.
  * .SE suffixes.
- CATEGORIES:
  * Data-dependent sorting.

Please try it out and let me know, especially if one of the features
not implemented is important to you or if you find a bug.