Fix CMake build instructions

[Arcnor]

This is a very simple PR to improve the CMake build instructions. The existing ones are a bit too convoluted and can be simplified a lot, plus there is an issue mentioned at #515 about the NVPTX using the wrong name, which leads to a misleading error. (The first issue mentioned there about the build type only shows when you configure CMake with a non-multi-config generator, so I've left that one alone for the time being).

I hope this helps!

[daniel347x-cs]

This simple PR seems super important to me. I wonder how many people, almost like myself, spending hours trying to follow the build instructions and failing, simply give up and don't use it or become involved.

Seems a shame not to make the very simple change of providing proper build instructions in the README that work in 2024.

[ferdymercury]

I believe this repo acts as a mirror, please move your PR here instead: https://github.com/root-project/root/pulls (subdirectory interpreter/cling). Thanks

[LiAuTraver]

Really spent a whole day failing finally found issue😭wish I could find this PR earlier but, until now neither the root nor this repo has fixed this instruction error, sadly it's half-year passed.

[ferdymercury]

Yes, it's a pity, but devs are quite overloaded afaik. This repo only gets (automatically) fixes from root, so this PR has more chances of getting reviewed if moved to the root repo.

[LiAuTraver]

Yes you are right. Plus i searched the PR/issue at root but not found any similiar problem. I create one myself root-project/root#15972 (comment). I really wish people come after do not ran into this problem, for it's a painful process building from source on an ordinary computer which results my PC was forced to shutdown several times. 😭

[ferdymercury]

Thanks for moving the PR there, I'll close this one then.