Can't always find aseba_INCLUDE_DIR, needed for examples

The examples try to explicitly find Aseba includes: https://github.com/aseba-community/aseba/blob/29b39bc39dde88f6d8f6489e7cf114d164c73e69/examples/CMakeLists.txt#L3-L5 This can fail in some build environments, such as Buildroot and MXE. In those cases it is necessary to supply e.g. -Daseba_INCLUDE_DIR=.. to CMake.

(I actually don't understand why examples/CMakeLists.txt, above, sets aseba_INCLUDE_DIR twice.)