build: hide doxygen commands with AM_V_GEN
build: hide doxygen commands with AM_V_GEN

I suppose the purpose was to print just one GEN line for each doxygen
rule being executed, not print the doxygen command.

Signed-off-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>

sir-murray/wayland,mchalupa/wayland,tulcod/wayland,bitfactor/wayland,bitfactor/wayland,bitfactor/wayland,eyolfson/wayland,abooij/wayland,mchalupa/wayland,abooij/wayland,sir-murray/wayland,eyolfson/wayland,abooij/wayland,sir-murray/wayland,tulcod/wayland,eyolfson/wayland,tulcod/wayland,mchalupa/wayland