Use pip and llvm-openmp

[jakirkham]

Use pip to build and install the package. Drop unneeded setting of LDFLAGS absorbed into compilers. Also make use of llvm-openmp for OpenMP support on macOS with the new compilers.

Checklist

• [x] Used a fork of the feedstock to propose changes
• [x] Bumped the build number (if the version is unchanged)
• [ ] Reset the build number to 0 (if the version changed)
• [ ] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [x] Ensured the license file is being packaged.

[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.

[jakirkham]

Saw some overlinkage warnings being raised, which don't look right. Have submitted upstream issue ( https://github.com/conda/conda-build/issues/3325 ).