Don't use g_thread_supported() with non-ancient glib versions

This is useless, as it always returns true (since 2.31), and just
results in warnings about using deprecated macro (since 2.62).

Also remove the now unnecessary warning suppression macros, as
g_thread_init() wasn't deprecated in 2.30 yet, so it shouldn't give any
warnings in glib versions in which it could be actually used.

src/gtk/app.cpp

@@ -276,6 +276,8 @@ bool wxApp::Initialize(int& argc_, wxChar **argv_)
     if ( !wxAppBase::Initialize(argc_, argv_) )
         return false;
 
+    // Thread support is always on since glib 2.31.
+#if !GLIB_CHECK_VERSION(2, 31, 0)
 #if wxUSE_THREADS
     if (!g_thread_supported())
     {
@@ -283,12 +285,11 @@ bool wxApp::Initialize(int& argc_, wxChar **argv_)
         // might still be needed in the older versions, which are the only ones
         // for which this code is going to be executed (as g_thread_supported()
         // is always TRUE in these recent glib versions anyhow).
-        wxGCC_WARNING_SUPPRESS(deprecated-declarations)
         g_thread_init(NULL);
-        wxGCC_WARNING_RESTORE()
         gdk_threads_init();
     }
 #endif // wxUSE_THREADS
+#endif // glib < 2.31
 
     // gtk+ 2.0 supports Unicode through UTF-8 strings
     wxConvCurrent = &wxConvUTF8;