[llvm] [mlir] [mlir][nvvm] Expose MLIR_NVPTXCOMPILER_ENABLED in mlir-config.h. (PR #84007)

[Ingo Müller via llvm-commits]

ingomueller-net wrote:

On first thought, that doesn't sound plausible. How are the two macros related? If you're using cmake, then it is most likely rather caused by #84011.

I am not sure about this specific case. Very generally, we either need to define the macro, either in a file or through the compiler/cmake/bazel, or turn the `#if` into `#ifdef`.

https://github.com/llvm/llvm-project/pull/84007