Ensure our include directories get added in the correct order

If you have assimp installed already and in the include path (e.g. I have it via homebrew), it can pick up the wrong headers.

This forces the include order so our local ones are found first when building assimp.

CMakeLists.txt

@@ -198,7 +198,7 @@ CONFIGURE_FILE(
   ${CMAKE_CURRENT_BINARY_DIR}/include/assimp/config.h
 )
 
-INCLUDE_DIRECTORIES(
+INCLUDE_DIRECTORIES( BEFORE
   ./
   include
   ${CMAKE_CURRENT_BINARY_DIR}