[llvm] [openmp] [offload][openmp] - Remove standalone build in favor of 'runtimes' (PR #170693)

[Michał Górny via llvm-commits]

mgorny wrote:

> The new 'standalone' runtimes method would require additional cmake options, LLVM_BINARY_DIR being one of them. Maybe those should be mentioned in the commit message. I wonder if a we should add a fatal error if it is not set.

But isn't `LLVM_BINARY_DIR` used as an output directory in a bunch of macros? I can try overriding it in a few minutes, but I do wonder if that's not going to cause the build system to try writing into the live system.

https://github.com/llvm/llvm-project/pull/170693