doc: do not rebuild man pages if they are up to date
doc: do not rebuild man pages if they are up to date

The doxygen.man make target was not a real file that was generated,
therefore the man page rule was ran on every make invocation. Replace it
with a real file that is produced by the man page rule.

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

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