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,
+static bool match_str(char *const str, regmatch_T *const regmatch, char **const matches,
-               const bool test_only, const bool fuzzy, const char *const fuzzystr,
+                      const int idx, const bool test_only, const bool fuzzy,
-               fuzmatch_str_T *const fuzmatch)
+                      const char *const fuzzystr, fuzmatch_str_T *const fuzmatch)
 {
   if (!fuzzy) {
     if (vim_regexec(regmatch, str, (colnr_T)0)) {

src/nvim/optionstr.c

@@ -676,8 +676,8 @@ static const char *did_set_option_listflag(char *val, char *flags, char *errbuf,
 }
 
 /// Expand an option that accepts a list of string values.
-int expand_set_opt_string(optexpand_T *args, char **values, size_t numValues, int *numMatches,
+static int expand_set_opt_string(optexpand_T *args, char **values, size_t numValues,
-                          char ***matches)
+                                 int *numMatches, char ***matches)
 {
   regmatch_T *regmatch = args->oe_regmatch;
   bool include_orig_val = args->oe_include_orig_val;
@@ -728,8 +728,8 @@ static char *expand_set_opt_callback(expand_T *xp, int idx)
 }
 
 /// Expand an option with a callback that iterates through a list of possible names.
-int expand_set_opt_generic(optexpand_T *args, CompleteListItemGetter func, int *numMatches,
+static int expand_set_opt_generic(optexpand_T *args, CompleteListItemGetter func, int *numMatches,
-                           char ***matches)
+                                  char ***matches)
 {
   set_opt_callback_orig_option = args->oe_include_orig_val ? args->oe_opt_value : NULL;
   set_opt_callback_func = func;
@@ -744,7 +744,7 @@ int expand_set_opt_generic(optexpand_T *args, CompleteListItemGetter func, int *
 }
 
 /// Expand an option which is a list of flags.
-int expand_set_opt_listflag(optexpand_T *args, char *flags, int *numMatches, char ***matches)
+static int expand_set_opt_listflag(optexpand_T *args, char *flags, int *numMatches, char ***matches)
 {
   char *option_val = args->oe_opt_value;
   char *cmdline_val = args->oe_set_arg;