I'm running LilyPond 2.18.2 on a Mac OS 10.8.5 and reported the same
error back in June:
http://lists.gnu.org/archive/html/lilypond-user/2014-06/msg00610.html
The problem (at least for me) seems to occur only when the --output or
--lily-output-dir options are used for lilypond-book. Take those
options away and lilypond-book finds the include files just fine. Use
one or the other (or both) and it complains. Using your MWE, I get the
same results.
Another list user (James) was able to replicate the issue with my MWE,
but no solution was proposed. I'd be very interested in any solution.