If you can’t use a more recent version of OCaml (configuration detection is much more robust starting from 4.08), you can try using ./configure -cc "gcc -Wno-implicit-function-declaration". I don’t think the maintainers will be interested in fixing the old configure scripts and make releases with those fixes, but if it feels important to you feel free too make a bug report (at least more people will be aware of the issue).
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