[pygobject] build: disable new cast-function-type warnings with gcc8

[Christoph Reiter <creiter src gnome org>]

commit dceb3e339149f0bc7e9f004632a6a24f38ca60ad
Author: Christoph Reiter <reiter christoph gmail com>
Date:   Thu May 3 11:34:01 2018 +0200

    build: disable new cast-function-type warnings with gcc8
    
    We have to assign PyCFunctionWithKeywords functions to PyCFunction fields
    everywhere, so not much we can do. Ideas welcome.

 meson.build | 1 +
 setup.py    | 1 +
 2 files changed, 2 insertions(+)
---
diff --git a/meson.build b/meson.build
index e2cf42b3..5e553766 100644
--- a/meson.build
+++ b/meson.build
@@ -89,6 +89,7 @@ main_c_args += [
   '-Wno-unused-parameter',
   '-Wno-discarded-qualifiers',
   '-Wno-sign-conversion',
+  '-Wno-cast-function-type',
 ]
 
 main_c_args += [
diff --git a/setup.py b/setup.py
index b6729eb2..b38a43e4 100755
--- a/setup.py
+++ b/setup.py
@@ -929,6 +929,7 @@ def add_ext_compiler_flags(ext, compiler, _cache={}):
             "-Wno-unused-parameter",
             "-Wno-discarded-qualifiers",
             "-Wno-sign-conversion",
+            "-Wno-cast-function-type",
         ]
 
         # silence clang for unused gcc CFLAGS added by Debian