Don't use old-style unprototyped C functions

[MisterDA]

Fixes strict-prototypes and old-style-definition warnings. As a reminder, f() declares f as an old-style function that can be applied to any number of arguments.

Subsumes and closes #11762.

Note: I've just noticed that I haven't enabled the warnings on mingw, so I haven't checked the prototypes on Windows only code yet.

[MisterDA]

Yes, there are more occurrences that I hadn't caught. Please give me a couple of minutes ;)

[xavierleroy]

Take your time :-)

[MisterDA]

If all is right, #11763 should show that this PR fixes all old-style prototypes on Linux, macOS and Windows since it adds warnings to catch them.
I've also had to remove K&R C style function definitions from Windows code in otherlibs/unix.

[shindere]

Awesome, @MisterDA!

[xavierleroy]

Looks great, thanks a lot! Just to make sure, I'm running a round of CI precheck to get feedback on all Windows ports. Will merge afterwards.

[MisterDA]

I've just force-pushed and applied this that I had forgotten:

diff --git a/testsuite/tests/asmgen/mainarith.c b/testsuite/tests/asmgen/mainarith.c
index ae4d1c50e0..fce523767a 100644
--- a/testsuite/tests/asmgen/mainarith.c
+++ b/testsuite/tests/asmgen/mainarith.c
@@ -22,7 +22,7 @@
 #include <caml/config.h>
 #define FMT ARCH_INTNAT_PRINTF_FORMAT

-void caml_call_poll()
+void caml_call_poll(void)
 {
 }

[xavierleroy]

CI precheck was happy, so I added a Changes entry and merged.