ITS#10011 build: fix compatibility with stricter C99 compilers

Fix the following warnings:
- -Wimplicit-int (fatal with Clang 16)
- -Wimplicit-function-declaration (fatal with Clang 16)
- -Wincompatible-function-pointer-types (fatal with Clang 16)
- -Wint-conversion (fatal with Clang 15)
- Old style prototypes (K&R, removed from C23)

These warnings-now-error led to misconfigurations and failure to build
OpenLDAP, as the tests used during configure caused the wrong results
to be emitted.

For more information, see LWN.net [0] or LLVM's Discourse [1], the Gentoo wiki [2],
or the (new) c-std-porting mailing list [3].

[0] https://lwn.net/Articles/913505/
[1] https://discourse.llvm.org/t/configure-script-breakage-with-the-new-werror-implicit-function-declaration/65213
[2] https://wiki.gentoo.org/wiki/Modern_C_porting
[3] hosted at lists.linux.dev.

Bug: https://bugs.gentoo.org/871288
Signed-off-by: Sam James <sam@gentoo.org>
