vim-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: vim/vim#13223
closes: vim/vim#13226

f7f746b167

Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>

src/nvim/option.c

@@ -5552,9 +5552,9 @@ void set_context_in_set_cmd(expand_T *xp, char *arg, int opt_flags)
 ///
 /// 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.
-bool match_str(char *const str, regmatch_T *const regmatch, char **const matches, const int idx,
-               const bool test_only, const bool fuzzy, const char *const fuzzystr,
-               fuzmatch_str_T *const fuzmatch)
+static bool match_str(char *const str, regmatch_T *const regmatch, char **const matches,
+                      const int idx, const bool test_only, const bool fuzzy,
+                      const char *const fuzzystr, fuzmatch_str_T *const fuzmatch)
 {
   if (!fuzzy) {
     if (vim_regexec(regmatch, str, (colnr_T)0)) {