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/optionstr.c

@@ -733,7 +733,7 @@ did_set_option_listflag(char_u *val, char_u *flags, char *errbuf)
 /*
  * Expand an option that accepts a list of string values.
  */
-    int
+    static int
 expand_set_opt_string(
 	optexpand_T *args,
 	char **values,
@@ -819,7 +819,7 @@ expand_set_opt_callback(expand_T *xp, int idx)
 /*
  * Expand an option with a callback that iterates through a list of possible names.
  */
-    int
+    static int
 expand_set_opt_generic(
 	optexpand_T *args,
 	char_u *((*func)(expand_T *, int)),
@@ -850,7 +850,7 @@ expand_set_opt_generic(
 /*
  * Expand an option which is a list of flags.
  */
-    int
+    static int
 expand_set_opt_listflag(
 	optexpand_T *args,
 	char_u *flags,