build: fix -Wimplicit-int (Clang 16)

Clang 16 will make -Wimplicit-int error by default which can, in addition to
other things, lead to some configure tests silently failing/returning the wrong result.

Fixes this error:
```
+/var/tmp/portage/media-libs/libvpx-1.12.0/temp/vpx-conf-1802-30624.c:1:15: error: type specifier missing, defaults to 'int'; ISO C99 and later do not support implicit int [-Wimplicit-int]
```

For more information, see LWN.net [0] or LLVM's Discourse [1], gentoo-dev@ [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://archives.gentoo.org/gentoo-dev/message/dd9f2d3082b8b6f8dfbccb0639e6e240
[3] hosted at lists.linux.dev.

Bug: https://bugs.gentoo.org/879705
Change-Id: Id73a98944ab3c99a368b9da7a5e902ddff9d937f
Signed-off-by: Sam James <sam@gentoo.org>
1 file changed