fix -Wreturn-type and -Wstrict-prototypes in gd configure

[morrisonlevi]

This kind of thing is why -Werror is enabled late, but in this case, I don't see any reason why we shouldn't just fix it. As C23 approaches, I expect more and more compilers to care about -Wstrict-prototypes and possibly even turn it to an error by default.

[Girgias]

I don't think GD can be compiled with -Wstrict-prototypes anyway, as some of the function pointers used need to be totally variadic as the number of parameters is not consistent.

Or is that a different warning?

[Girgias]

Other than the general comment, this looks sensible to me

[morrisonlevi]

I hit this when using some other flag combinations because I didn't even get through the configure stage. I'm not sure about the gd prototype stuff, but this failure is in a random config.m4 check written ages ago.

[morrisonlevi]

Fun, there's a segmentation fault in ext/oci8/tests/conn_attr_4.phpt: https://github.com/php/php-src/actions/runs/6358055629/job/17269921241?pr=12328

Of course, there's no way this is the fault of this PR. Is there someone specifically I should ping?

[ndossche]

@morrisonlevi That was due to a bad commit in 8.3 that's been reverted in the meantime. Rebasing your PR should fix it.