Fix CMake for FetchContent usecase and add integration test

The recent change to target_include_directories with ${CMAKE_SOURCE_DIR} caused the integration to fail. Using CMAKE_CURRENT_SOURCE_DIR instead is not a solution either, since it returns the current directory (as the name implies), not the top-level source directory. The simplest fix was to fix back to using a global include_directories directive. An integration test has been added to the CI to test this functionality.