I was building ardour-2.1 from source and got something like this :
$ scons GTKOSX=1 COREAUDIO=1 AUDIOUNITS=1 FFT_ANALYSIS=1 SURFACES=1
error : libs/ardour/plugin.cc:297 : AUPluginInfo is not declared in this scope.
AUPluginInfo is declared in “ardour/audio_unit.h”
which is not included in plugin.cc
so I had #include “ardour/audio_unit.h” and the compilation is carrying on (for the moment at least).
I don’t know if this was on purpose and hopes it can helps developers.