Using the Debug build in vs2010 is not working because the debug dlls are
not loaded when debugging. The reason they are not loaded is that
CMAKE_BUILD_TYPE is not defined when doing multiple builds. This in turns
causes OGRE_PLUGIN_DEBUG_SUFFIX not to be set. This patch makes sure that
OGRE_PLUGIN_DEBUG_SUFFIX is always set but only used when debugging.
There are still other bugs that have broken Debug mode in vs2010 but those
will be addressed in other patches.
