Add c99 flag for gcc try 2

[WilliamJamieson]

This PR is attempt 2 at the fix attempted by #199

Checklist

• [ ] added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• [ ] updated relevant tests
• [ ] updated relevant documentation
• [ ] updated relevant milestone(s)
• [ ] added relevant label(s)

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (507cddd) 82.31% compared to head (cdaef95) 82.31%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #201 +/- ## ======================================= Coverage 82.31% 82.31% ======================================= Files 29 29 Lines 5554 5554 ======================================= Hits 4572 4572 Misses 982 982 ``` | [Files Changed](https://app.codecov.io/gh/spacetelescope/stcal/pull/201?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope) | Coverage Δ | | |---|---|---| | [setup.py](https://app.codecov.io/gh/spacetelescope/stcal/pull/201?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope#diff-c2V0dXAucHk=) | `0.00% <0.00%> (ø)` | |

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