configure.ac: drop "auto" detection of doxygen dependencies
configure.ac: drop "auto" detection of doxygen dependencies

Always build the documentation, if someone doesn't want it disable it with a
configure flag but at least be explicit about it.

Justification: any developer should build the full stack as intended by
default. Distributions may disable bits and pieces, but they shouldn't rely on
auto-detection to figure it out, they should explicity disable those bits they
don't want.

Signed-off-by: Peter Hutterer <b9c50af7afa7642f6f854434cda9ffe54ece8b85@who-t.net>

libratbag/libratbag,whot/libratbag,libratbag/libratbag,libratbag/libratbag,whot/libratbag,whot/libratbag