CMake subproject support; related to #1477

This changes CMAKE_SOURCE_DIR to CMAKE_CURRENT_SOURCE_DIR to allow graphviz to be built as a CMake subproject; see #1477 (closed).

I scanned for use of CMAKE_SOURCE_DIR and CMAKE_BINARY_DIR in all CMakeLists.txt files and only found CMAKE_SOURCE_DIR used in the top-level CMakeLists.txt. I tested by building a simple project in the parent directory of the graphviz project root using this simple CMakeLists.txt config. The project built without obvious errors on Windows and the change does not seem to affect the normal build process.