require c99 standard compiler to build cfe and topmodel libs

[madMatchstick]

Invalid syntax causes build errors for compilers <c99.

for( int i = 0; i < INPUT_VAR_NAME_COUNT; i++){

}

See initial conversion in submodule evapotranspiration.

Additions

• Make C99 standard required for building external topmodel and cfe libraries.

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️

Testing checklist (automated report can be put here)

Target Environment support

• Linux