Support for building on OpenBSD

With dependencies (ois, ogre) installed from the OpenBSD packages and these changes, I was able to build mygui on OpenBSD 5.6-current amd64 without problems. Most of the demos run without problems.

Demo_PluginStrangeButton core dumps with the following message. This makes me unsure whether my change to PluginStrangeButton's CMakeLists is correct:

~~~
Loading library libPlugin_StrangeButton.so
Plugin 'libPlugin_StrangeButton.so' not found
factory 'StrangeButton' not found
An exception has occured : MyGUI EXCEPTION : factory 'StrangeButton' not found
 in MyGUI at /home/user/mygui/MyGUIEngine/src/MyGUI_WidgetManager.cpp (line 115)terminate called after throwing an instance of 'MyGUI::Exception'
  what():  MyGUI EXCEPTION : factory 'StrangeButton' not found
 in MyGUI at /home/user/mygui/MyGUIEngine/src/MyGUI_WidgetManager.cpp (line 115)
Abort trap (core dumped)
~~~