doc: apply a bunch of makefile fixes

Gaetan's makefile fixes for libevdev/doc apply here too. For more reasoning
see libevdev's tree for
    doxygen: libevdev.doxygen must not be distributed
    doxygen: invocation of doxygen fails due to hard coded path
    doxygen: implement silent rules in the makefile
    doxygen: remove redundant "dist_noinst_DATA = libevdev.doxygen.in" statement
    doxygen: fix error msg: 'find: `html': No such file or directory'
    doxygen: fix rebuild when not necessary
    doxygen: fix 'dist' when the html docs have not yet been built

libevdev commits a742d253b72b6c..c495cb42c04252

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