Fix CMakeLists.txt variable reference to prevent full system recursion

[adamconkey]

The variable dbot_SOURCE_DIR referenced in CMakeLists.txt is not set in this file. As such, it takes it as an empty string, and you end up recursing on the ENTIRE filesystem. Kind of awesome, it was looking for headers in my trash and in other user accounts.

The fix is simply use dbot_ros_SOURCE_DIR instead. An added benefit is it takes significantly less time to compile now.