Made CMake include paths relative.

Replaced `CMAKE_SOURCE_DIR` with `CMAKE_CURRENT_SOURCE_DIR` and
`CMAKE_BINARY_DIR` with `CMAKE_CURRENT_BINARY_DIR`. This allows to use
OGS as a subdirectory in another project like this:

- Create a new CMake project
- Clone the ogs repo into the new project as subdirectory (e.g. via `git submodule`)
- Add the ogs dir as a subdirectory:

    ```cmake
CMAKE_MINIMUM_REQUIRED(VERSION 2.8.7)
PROJECT( OGS-6-Superbuild )
ADD_SUBDIRECTORY(${CMAKE_SOURCE_DIR}/ogs)
    ```