Allow using versions of libnotify < 0.7 in wxGTK.

Allow using versions of libnotify < 0.7 in wxGTK.

libnotify 0.4.5 included in Ubuntu Lucid (10.04) can be used for implementing
wxNotificationMessage too, there is only a minor difference with 0.7 API. So
while we stick check for 0.7 initially, accept lower versions too if 0.7 is
not available.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72456 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775