* CMake: use PROJECT_VERSION instead of solvespace_*_VERSION
In 006539b, solvespace_MAJOR_VERSION etc. were removed.
However, these variables were still referenced in some places.
Solution: Use PROJECT_VERSION instead.
* CMake: re-add link directories for solvespace target
006539b removed the call to link_directories for gtkmm, jsonc & fontconfig.
This leads to linking errors if those libraries are in "non-standard"
paths.
Fix this by introducing a target specific target_link_directories call.
Fixes#1173
* snap: port to core20 & adapt to CMake changes
Moving to core20 was long overdue anyway, and
the recent CMake changes necessitated some fixes.
Also switch to LZO compression for (way) better cold start
performance.
Maximilian Federle
whitequark