glib: use portable g_setenv()

We have a setenv() wrapper in os-win32.c that no one is actually using.
Drop it and change to g_setenv() uniformly.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1576074210-52834-7-git-send-email-pbonzini@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
diff --git a/ui/sdl2.c b/ui/sdl2.c
index bd4e736..9030f1c 100644
--- a/ui/sdl2.c
+++ b/ui/sdl2.c
@@ -772,7 +772,7 @@
      * This is a bit hackish but saves us from bigger problem.
      * Maybe it's a good idea to fix this in SDL instead.
      */
-    setenv("SDL_VIDEODRIVER", "x11", 0);
+    g_setenv("SDL_VIDEODRIVER", "x11", 0);
 #endif
 
     if (SDL_Init(SDL_INIT_VIDEO)) {