Fix CMake scripts to link static dependencies

Added FindDirectX scripts from Ogre so that FindOgre.cmake doesn't
fail silently to find the libraries correctly.

Modified top-level CMakelists.txt:
- Correctly set include dirs, preprocessor definitions, and add
  libraries for static Ogre plugins.
- Correctly set preprocessor definitions for static MyGUI.

Modified launcher, openmw, and opencs to link the static Ogre plugins.

Fixed FindSDL2.cmake to set a variable that contains only the SDL2
library, so that the launcher doesn't incorrectly link against
SDL2main.

Fixed FindMyGUI.cmake to correctly look for static variants.

Fixed FindOgre.cmake to set variables for DirectX plugins correctly.