Adapt build system to allow use of WebODF as part of a larger CMake project.

Adapt build system to allow use of WebODF as part of a larger CMake project.
All instances of CMAKE_BINARY_DIR and CMAKE_SOURCE_DIR are replaced with CMAKE_CURRENT_BINARY_DIR CMAKE_CURRENT_SOURCE_DIR. CMAKE_BINARY_DIR and CMAKE_SOURCE_DIR point to the project root, which is different when WebODF is used in a larger project.