We said that some of our dependencies were optional, but didn't say
how to turn them off.  Add information for that and mention where to
save the options close to the top of the file.

Also, standardize on both using quotes for the names of the dependencies
and tabs for indentation of the list.
Signed-off-by: NBrian Gernhardt <brian@gernhardtsoftware.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>