openmp 12.0.1 (redux)

[h-vetinari]

Closes #48
Closes #47

I tried rebasing the failing patch, but it looks like it has become obsolete since llvm/llvm-project@7b52542

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[h-vetinari]

Looking at https://github.com/llvm/llvm-project/blame/release/12.x/openmp/runtime/src/kmp.h, I have a suspicion that llvm/llvm-project@e0665a9 has broken the openmp windows build, because of improperly guarded __asm__.

Don't remember off the top of my head if #include <intrin.h> would help, but based on the logs, it's only looking for immintrin.h, and I don't think the guard is kicking in.

[...]
-- Looking for pthread.h
-- Looking for pthread.h - not found
-- Found Threads: TRUE  
-- Looking for immintrin.h
-- Looking for immintrin.h - found
-- Performing Test LIBOMP_HAVE_ATTRIBUTE_RTM
-- Performing Test LIBOMP_HAVE_ATTRIBUTE_RTM - Failed
[...]

[isuruf]

We should just use clang-cl as the compiler for windows.