Fix empty includes in add_pFUnit_executable.

It's possible for the target created by add_pFUnit_executable to have a
non-existent INCLUDE_DIRECTORIES property, meaning that get_target_property
will set the "includes" variable to NOTFOUND.

We need to check for that case and unset the variable if this happens;
otherwise the string "NOTFOUND" can end up in the INCLUDE_DIRECTORIES and
cause an error.