[flang] [llvm] [flang] Generate quadmath_wrapper.h for Flang Evaluate. (PR #132817)

[Michael Kruse via llvm-commits]

https://github.com/Meinersbur approved this pull request.

LGTM for now, but in the mid-term we **must** absolutely avoid having compiler behavior depend on the system configuration where it was compiled. Think of e.g. Debian not having quadmath configured on their build server and because of that no Debian user can compile code with `REAL(16)` in it.

> I extracted the CMake code into FlangCommon.cmake, and cleaned up the arguments passing to execute_process (note that -### was treated as - in the original code, because # starts a comment).

Thanks for fixing.

https://github.com/llvm/llvm-project/pull/132817