Compiler flag improvements

[brycelelbach]

Turn on warnings and fix some subsequent issues, add a bunch of compiler diagnostic flags, tweak build mode flags.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (356a2ad) 97.86% compared to head (f0ea7f0) 97.86%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #93 +/- ## ======================================= Coverage 97.86% 97.86% ======================================= Files 65 65 Lines 2201 2201 ======================================= Hits 2154 2154 Misses 47 47 ```

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[tcbrindle]

Thanks for this Bryce! My CMake-fu is pretty weak so thanks for having a go at improving it

A couple of quick questions:

• Can the flux_docs library be defined in examples/CMakeLists.txt instead, to keep things tidier in the top-level file? (Are there many examples which are causing unused variable warnings?)
• I'm a little bit concerned about making decisions regarding optimisation settings (-fomit-frame-pointer etc) on the top-level flux library that would propagate to downstream users who might necessarily not want them... would it be better to add e.g. an empty flux_internal interface library that we could set all the optimisation flags for, and then have the test/benchmark executables use that?

[tcbrindle]

@brycelelbach I'm going to close this as it's been a while and there have been quite a lot of CMake changes since. Feel free to submit another PR if you're still interested :)