libXaw: do not build docs (fails if missing host tools), also cleanup makefile

Signed-off-by: Will Wagner <will_wagner@carallon.com>
Acked-by: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>