This re-introduces the apidoc with full config.h, but hopefully with the race
conditions and other issues that the previous implementation had.
Adapt doxygen test script to use that new script, and also check for errors
in addition to warnings while at it.
Apparently travis has an old version of doxygen that doesn't know all tags in
our config. That's not something we care about, we only want to know about
warnings in our doxygen content