Will not build with g++ 9.3

[tukkek]

gui/csg_clip.cc: In function ‘void DoWriteClip(dclipnode_t&, bool)’:
gui/csg_clip.cc:1057:32: error: cannot bind packed field ‘raw_clip.dclipnode_s::children[0]’ to ‘short unsigned int&’
 1057 |   std::swap(raw_clip.children[0], raw_clip.children[1]);
      |             ~~~~~~~~~~~~~~~~~~~^
make: *** [Makefile:106: obj_linux/csg_clip.o] Error 1

Thanks to this comment though, I have been able to properly compile Oblige after installing g++ 8 8.4. This can easily be done by changing this line to:

CXX=g++-8

My suggestions towards solving this problem:

1. Fix the compilation error(s) to work with the latest version of g++; or...
2. Update the documentation files and Makefile to officially and explicitly depend on g++ 8.

Given that the second approach is a simple-enough task and within my technical knowledge. I'd be happy to draft a pull request for that. However, I'd imagine the first option is probably the more future-proof way of addressing the issue.