Add WXSUPPRESS_GTK_DIAGNOSTICS environment variable

This allows to suppress some or all GTK diagnostics even for the
programs not calling GTKSuppressDiagnostics() nor even
GTKAllowDiagnosticsControl() themselves.

docs/doxygen/overviews/envvars.h

@@ -45,6 +45,12 @@ wxWidgets programs.
          This can be helpful when running older programs recompiled with
          wxWidgets 3.1 or later, as these asserts are mostly harmless and can
          be safely ignored if the code works as expected.}
+@itemdef{WXSUPPRESS_GTK_DIAGNOSTICS,
+         If set to a non-zero value, wxApp::GTKSuppressDiagnostics() is called
+         on program startup using the numeric value of this variable or the
+         default value if it's not a number, so that e.g. setting it to "yes"
+         suppresses all GTK diagnostics while setting it to 16 only suppresses
+         GTK warning messages.}
 */
 
 @see wxSystemOptions

interface/wx/app.h

@@ -1042,6 +1042,11 @@ public:
         This function can be called to suppress GTK diagnostic messages that
         are output on the standard error stream by default.
 
+        If @c WXSUPPRESS_GTK_DIAGNOSTICS environment variable is set to a
+        non-zero value, wxWidgets automatically calls this function on program
+        startup with the value of this variable as @a flags if it's a number or
+        with the default flags value otherwise.
+
         The default value of the argument disables all messages, but you
         can pass in a mask flag to specifically disable only particular
         categories of messages.

src/gtk/app.cpp

@@ -392,6 +392,19 @@ bool wxApp::OnInitGui()
     }
 #endif
 
+    // Suppress GTK diagnostics if requested: this is convenient if the program
+    // doesn't use GTKAllowDiagnosticsControl() itself.
+    wxString suppress;
+    if ( wxGetEnv("WXSUPPRESS_GTK_DIAGNOSTICS", &suppress) )
+    {
+        long flags;
+        if ( !suppress.ToLong(&flags) )
+            flags = -1; // Suppress everything by default.
+
+        if ( flags != 0 )
+            GTKSuppressDiagnostics(flags);
+    }
+
     return true;
 }