Test whether GTK+ is 2.18 or newer in configure.

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