Test whether GTK+ is 2.18 or newer in configure.

We need to define __WXGTK218__ symbol in configure instead of simply checking
if we are compiled with 2.18 in wx/infobar.h because GTK_CHECK_VERSION() can't
be used when compiling user code which can't even include gtk/gtkversion.h
defining it because it doesn't necessarily use `pkg-config --cflags gtk+-2.0`
at all in its CFLAGS.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62405 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/infobar.h

@@ -52,12 +52,9 @@ private:
 };
 
 // currently only GTK+ has a native implementation
-#if defined(__WXGTK20__) && !defined(__WXUNIVERSAL__)
-    #include <gtk/gtkversion.h>
-    #if GTK_CHECK_VERSION(2, 18, 0)
-        #include "wx/gtk/infobar.h"
-        #define wxHAS_NATIVE_INFOBAR
-    #endif
+#if defined(__WXGTK218__) && !defined(__WXUNIVERSAL__)
+    #include "wx/gtk/infobar.h"
+    #define wxHAS_NATIVE_INFOBAR
 #endif // wxGTK2
 
 // if the generic version is the only one we have, use it