Introduced a OSG_PLUGIN_PREFIX variable into CMake build system with it set to

"" for all platforms except Cygwin where its set to "cygwin_" and Mingw where
it is set to "mingw_".  Updated osgDB::Registry to look for these for the plugins.
Updated the osgintrospection example to search for these names as well.

examples/osgintrospection/osgintrospection.cpp

@@ -38,9 +38,9 @@ std::string createLibraryNameForWrapper(const std::string& ext)
 #if defined(WIN32)
     // !! recheck evolving Cygwin DLL extension naming protocols !! NHV
     #ifdef __CYGWIN__
-        return "cygosgwrapper_"+ext+".dll";
+        return "cygwin_osgwrapper_"+ext+".dll";
     #elif defined(__MINGW32__)
-        return "libosgwrapper_"+ext+".dll";
+        return "mingw_osgwrapper_"+ext+".dll";
     #else
         #ifdef _DEBUG
             return "osgwrapper_"+ext+"d.dll";