Fix function signature casts in PyMethodDef structs that lead to warnings in...

Fix function signature casts in PyMethodDef structs that lead to warnings in gcc-8 (and actually make use of the warnings in gcc-8).

Cython/Compiler/Code.py

@@ -2193,10 +2193,18 @@ class CCodeWriter(object):
         if method_flags:
             if entry.is_special:
                 method_flags += [method_coexist]
+            func_ptr = entry.func_cname
+            # Add extra casts, but try not to shadow real warnings.
+            cast = '__Pyx_PyCFunctionFast' if 'METH_FASTCALL' in method_flags else 'PyCFunction'
+            if 'METH_KEYWORDS' in method_flags:
+                # Need an extra cast
+                cast += 'WithKeywords'
+            if cast != 'PyCFunction':
+                func_ptr = '(void*)(%s)%s' % (cast, func_ptr)
             self.putln(
                 '{"%s", (PyCFunction)%s, %s, %s}%s' % (
                     entry.name,
-                    entry.func_cname,
+                    func_ptr,
                     "|".join(method_flags),
                     doc_code,
                     term))