Try with clang

[jaimergp]

Checklist

• Used a fork of the feedstock to propose changes
• 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 @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

Trying clang to workaround issues found in #23 due to GCC 8.5

[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.

[jaimergp]

@isuruf, clang works in every platform but ppc64le. It complains about the -mpower8-fusion flag. Any ideas?

[jaimergp]

If I disable -mpower8-fusion, it starts building successfully, but then we reach an error concerning PPC intrinsics:

In file included from /home/conda/feedstock_root/build_artifacts/openmm_1591177558831/work/platforms/cpu/src/CpuCustomGBForce.cpp:30:
In file included from /home/conda/feedstock_root/build_artifacts/openmm_1591177558831/work/platforms/cpu/./include/CpuCustomGBForce.h:33:
In file included from /home/conda/feedstock_root/build_artifacts/openmm_1591177558831/work/openmmapi/include/openmm/internal/vectorize.h:38:
/home/conda/feedstock_root/build_artifacts/openmm_1591177558831/work/openmmapi/include/openmm/internal/vectorize_ppc.h:286:20: error: use of overloaded operator '+' is ambiguous (with operand types 'fvec4' and '__vector float' (vector of 4 'float' values))
    fvec4 temp = r + vec_sld(r.val, r.val, 8);
                 ~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~
/home/conda/feedstock_root/build_artifacts/openmm_1591177558831/work/openmmapi/include/openmm/internal/vectorize_ppc.h:100:11: note: candidate function
    fvec4 operator+(fvec4 other) const {
          ^
/home/conda/feedstock_root/build_artifacts/openmm_1591177558831/work/openmmapi/include/openmm/internal/vectorize_ppc.h:286:20: note: built-in candidate operator+(__vector float, __vector float)
    fvec4 temp = r + vec_sld(r.val, r.val, 8);
                   ^

This might be also the cause behind the force discrepancies with GCC?

[jaimergp]

Not needed now that we have #23 working