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_py_both.h

@@ -105,7 +105,7 @@ struct typeobject_wrapper {
 # define Py_TYPE_GET_TP_METHODS(type) ((PyMethodDef *)PyType_GetSlot(type, Py_tp_methods))
 
 // PyObject_NEW is not part of stable ABI, but PyObject_Malloc/Init are.
-PyObject* Vim_PyObject_New(PyTypeObject *type, size_t objsize)
+static PyObject* Vim_PyObject_New(PyTypeObject *type, size_t objsize)
 {
     PyObject *obj = (PyObject *)PyObject_Malloc(objsize);
     if (obj == NULL)
@@ -168,7 +168,7 @@ PyObject* Vim_PyObject_New(PyTypeObject *type, size_t objsize)
 #  define PyIter_Check(obj) (FALSE)
 # endif
 
-PyTypeObject* AddHeapType(struct typeobject_wrapper* type_object)
+static PyTypeObject* AddHeapType(struct typeobject_wrapper* type_object)
 {
     PyType_Spec type_spec;
     type_spec.name = type_object->tp_name;
@@ -254,7 +254,7 @@ PyTypeObject* AddHeapType(struct typeobject_wrapper* type_object)
 
 // Limited API does not provide PyRun_* functions. Need to implement manually
 // using PyCompile and PyEval.
-PyObject* Vim_PyRun_String(const char *str, int start, PyObject *globals, PyObject *locals)
+static PyObject* Vim_PyRun_String(const char *str, int start, PyObject *globals, PyObject *locals)
 {
     // Just pass "" for filename for now.
     PyObject* compiled = Py_CompileString(str, "", start);
@@ -265,7 +265,7 @@ PyObject* Vim_PyRun_String(const char *str, int start, PyObject *globals, PyObje
     Py_DECREF(compiled);
     return eval_result;
 }
-int Vim_PyRun_SimpleString(const char *str)
+static int Vim_PyRun_SimpleString(const char *str)
 {
     // This function emulates CPython's implementation.
     PyObject* m = PyImport_AddModule("__main__");