Simplify logic for defining the nowarn macro.

[dholladay00]

PR Summary

I am unable to install singularity-eos from spack. This may be a combination of new ports-of-call and overly complicated logic for defining the SG_PIF_NOWARN macro. This simplifies that preprocessor logic that I believe was overly complex.

If this passes tests, I think we should merge it quickly @Yurlungur @jhp-lanl @rbberger

PR Checklist

• Adds a test for any bugs fixed. Adds tests for new features.
• Format your changes by using the make format command after configuring with cmake.
• Document any new features, update documentation for changes made.
• Make sure the copyright notice on any files you modified is up to date.
• After creating a pull request, note it in the CHANGELOG.md file
• If preparing for a new release, update the version in cmake.

[jhp-lanl]

I don't fully understand the problem this is fixing, but this does seem simpler and more targeted.

@annapiegra even if this gets merged, do you have opinions?

[jhp-lanl]

For posterity, another explanation from @dholladay00 :

There exists a bug in how that macro is defined in which it won't get defined in all cases and it must be defined in all cases or else it won't compile. This PR fixes said bug. In my case, I observed this bug when trying to install via spack so that I could work on getting singularity up to date with newer poc.