[clang] [clang/www/get_started.html] Use newer `cmake` syntax (PR #93503)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

In general, I think this is heading in the right direction. I did have a suggestion that may be worth considering.

https://github.com/llvm/llvm-project/pull/93503