Make doxygen doc generation optional.
Make doxygen doc generation optional.

Make doxygen doc generation optional at configure time instead of being
automagically detected. If doc generation is explicitely requested but
doxygen is available then configure will abort with an error.

reverendhomer/libmtp,olunx/libmtp,Bluerise/libmtp,philipl/libmtp,philipl/libmtp,Bluerise/libmtp,reverendhomer/libmtp,olunx/libmtp,yifanlu/libMTP,libmtp/libmtp,mmalecki/libmtp,yifanlu/libMTP,reverendhomer/libmtp,jemc/libmtp,libmtp/libmtp,reverendhomer/libmtp,libmtp/libmtp,mmalecki/libmtp,jemc/libmtp,olunx/libmtp,kbhomes/libmtp-zune,vmandela/libmtp,vmandela/libmtp,pierrezurek/libmtp,philipl/libmtp,kbhomes/libmtp-zune,pierrezurek/libmtp,mmalecki/libmtp,jemc/libmtp,jemc/libmtp,Bluerise/libmtp