Make the INSTALL(DIRECTORY empty) calls work with CMake >= 3.1.0.

Since CMake 3.1.0, `${LOG_DIR}`, `${PID_DIR}` and `${WORK_DIR}` were no longer
being created because the regular expression does not match anything and
CMake's behavior changed in a way that those directories would not be
created before filtering the match list anymore.

Instead, just call `INSTALL(DIRECTORY)` with an empty source, which works
with all CMake versions currently supported.