patch 9.0.1960: Make CI checks more strict

Problem:  Make CI checks more strict
Solution: Add -Wstrict-prototypes -Wmissing-prototypes to CI,
          fix uncovered problems

Add -Wstrict-prototypes -Wmissing-prototypes warnings check to CI

Add two new warnings to CI, silence some Perl related build-warnings:

- `strict-prototypes` helps prevent declaring a function with an empty
  argument list, e.g. `int func()`. In C++, that's equivalent to `int
  func(void)`, but in C, that means a function that can take any number
  of arguments which is rarely what we want.

- `missing-prototypes` makes sure we use `static` for file-only internal
  functions. Non-static functions should have been declared on a
  prototype file.

- Add `no-compound-token-split-by-macro` to the perl cflags, since it
  throws out a bunch of perl-related warnings that make the CI log
  unnecessary verbose and hard to read. This seems to happen only with
  clang 12 and above.

When applying those changes, it already uncovered a few warnings, so fix
up the code as well (fix prototypes, make the code static, remove
shadowed var declaration)

GTK header needs to have #pragma warning suppressiong because GTK2
headers will warn on `-Wstrict-prototypes`, and it's included by gui.h
and so we can't just turn off the warning in a couple files.

closes: #13223
closes: #13226

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>

src/option.c

@@ -7518,7 +7518,7 @@ set_context_in_set_cmd(
  * If 'test_only' is FALSE and 'fuzzy' is TRUE and if 'str' fuzzy matches
  * 'fuzzystr', then stores the match details in fuzmatch[idx] and returns TRUE.
  */
-    int
+    static int
 match_str(
 	char_u		*str,
 	regmatch_T	*regmatch,
@@ -8014,7 +8014,7 @@ ExpandSettingSubtract(
 	    // character as individual choice.
 	    for (char_u *flag = option_val; *flag != NUL; flag++)
 	    {
-		char_u *p = vim_strnsave(flag, 1);
+		p = vim_strnsave(flag, 1);
 		if (p == NULL)
 		    break;
 		(*matches)[count++] = p;