build: enforce out-of-tree builds

[jengelh]

https://github.com/kraflab/dsda-doom/pull/446#issuecomment-1817661300

cmake and make should be ran inside a build folder, not from /prboom2

[fabiangreffrath]

We have a much shorter variant of this in Woof:

https://github.com/fabiangreffrath/woof/blob/0f49f788bdad5a66a6290c14e6eb9c4f4c0956c0/CMakeLists.txt#L32-L35

[kraflab]

Please adopt Woof's implementation if it also works.

[kraflab]

Closing this as it's now nearly 4 months old, but feel free to revive it in the future.