gh-99086: Further -Wstrict-prototypes, -Wimplicit-function-declaration fixes

Followup to 12078e7. I ended up missing
a few, apparently.

Clang 16 makes -Wimplicit-function-declaration an error by default
and warns on -Wstrict-prototypes. It's free to fix -Wstrict-prototypes
while we're here and it futureproofs us, so throw that in.

The only notable part of this which might not be super obvious is
<stddef.h> for size_t.

See: 12078e7
Signed-off-by: Sam James <sam@gentoo.org>

Author: thesamesam

Misc/NEWS.d/next/Build/2022-11-04-02-58-10.gh-issue-99086.DV_4Br.rst

@@ -1 +1 @@
-Fix ``-Wimplicit-int`` compiler warning in :program:`configure` check for ``PTHREAD_SCOPE_SYSTEM``.
+Fix ``-Wimplicit-int``, ``-Wstrict-prototypes``, and ``-Wimplicit-function-declaration`` compiler warnings in :program:`configure` checks.