boost: Add comment when C++ is missing

[Peter: move above boost option so sub options gets indented]
Signed-off-by: Stephan Thamm <thammi@chaossource.net>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>