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/if_python3.c

@@ -395,8 +395,8 @@ static int (*py3_PyObject_SetAttrString)(PyObject *, const char *, PyObject *);
 static PyObject* (*py3_PyObject_CallFunctionObjArgs)(PyObject *, ...);
 static PyObject* (*py3__PyObject_CallFunction_SizeT)(PyObject *, char *, ...);
 static PyObject* (*py3_PyObject_Call)(PyObject *, PyObject *, PyObject *);
-static PyObject* (*py3_PyEval_GetGlobals)();
-static PyObject* (*py3_PyEval_GetLocals)();
+static PyObject* (*py3_PyEval_GetGlobals)(void);
+static PyObject* (*py3_PyEval_GetLocals)(void);
 static PyObject* (*py3_PyList_GetItem)(PyObject *, Py_ssize_t);
 static PyObject* (*py3_PyImport_ImportModule)(const char *);
 static PyObject* (*py3_PyImport_AddModule)(const char *);
@@ -1061,7 +1061,7 @@ static struct PyModuleDef vimmodule;
 // An alternative would be to convert all attribute string comparisons to use
 // PyUnicode_CompareWithASCIIString to skip having to extract the chars.
 static char py3_unicode_utf8_chars[20];
-char* PY_UNICODE_GET_UTF8_CHARS(PyObject* str)
+static char* PY_UNICODE_GET_UTF8_CHARS(PyObject* str)
 {
     py3_unicode_utf8_chars[0] = '\0';
     PyObject* bytes = PyUnicode_AsUTF8String(str);