build: Add compiler-dependent flag setup from libCEED Makefile

James Wrightrequested to merge
jrwrigh/makefile_additions into main

Summary

Takes the compiler-dependent flags from libCEED and adds them here. This has the following effects:

  • Better default performance compiling wise (automatically adds in the appropriate SIMD and -march flags)
  • Adds in some new warning flags

The new warning flags uncovered some issues previously, so those are fixed in this PR as well.

Checklist

  • Atomic commit history
  • All new features documented
  • Updated CHANGELOG.md

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