Update CMakeLists.txt

Fix for https://github.com/assimp/assimp/issues/166

code/CMakeLists.txt

@@ -656,6 +656,11 @@ if ( MSVC )
   ADD_DEFINITIONS( -D_CRT_SECURE_NO_WARNINGS )
 endif ( MSVC )
 
+if (APPLE)
+  SET_TARGET_PROPERTIES( assimp PROPERTIES
+    INSTALL_NAME_DIR "${CMAKE_INSTALL_PREFIX}/${LIB_INSTALL_DIR}"
+  )
+endif()
 if (UNZIP_FOUND)
   SET (unzip_compile_SRCS "")
 else (UNZIP_FOUND)