acinclude.m4: fix configure tests broken with Clang 15 (implicit func…

[thesamesam]

…tion declarations)

Clang 15 makes implicit function declarations fatal by default which leads to some configure tests silently failing/returning the wrong result.

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

[thesamesam]

@tats any chance you could take a look? thanks!

[tats]

Merged, thanks for your contribution.

[thesamesam]

Thanks!