Fix configure.ac compatibility with Clang 16

[thesamesam]

Clang 16 makes -Wimplicit-function-declaration and -Wimplicit-int errors by default.

Unfortunately, this can lead to misconfiguration or miscompilation of software as configure tests may then return the wrong result.

We also fix -Wstrict-prototypes while here as it's easy to do and it prepares us for C23.

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/881713 Signed-off-by: Sam James [email protected]