C implementation: Fix compiler warnings

[fulldecent]

The C implementation currently has compiler warnings.

https://github.com/google/open-location-code/actions/runs/12468691641/job/34800362797

These are related to non-standard extensions implemented in our C implementation instead of what our specification requires.

[fulldecent]

Fixed at #651

That PR

• Fixes C build warnings from code issue
• Fixes C build warnings from Bazel issues

and results in no C build warnings

[drinckes]

What are the warnings you want to address? Is it just?

warning: comparison of integer expressions of different signedness: 'int' and 'long unsigned int' [-Wsign-compare]

[fulldecent]

I wanted it to build clean with no warnings or errors as reported in GitHub actions