[CMake] FindwxWidgets and wxpng

[James Bigler]

In windows it looks for the library path by looking for the wxpng library:

       FIND_PATH(wxWidgets_LIB_DIR
         NAMES wxpng.lib wxpngd.lib
         PATHS
         ${WX_ROOT_DIR}/lib/vc_lib   # prefer static
         ${WX_ROOT_DIR}/lib/vc_dll
         DOC "Path to wxWidgets libraries?"
         NO_DEFAULT_PATH
         )

There's nothing in the documentation that states I have to have compiled 
wxpng for this to work.  It seems like it should be using at least one 
of the libraries that I asked it to look for (which didn't include 
wxpng), such as base or core or mono.

I'm using cmake 2.4.7 on win XP with wxWidgets 2.8.7.

James