manual: refactor the targets into GENDOC macro.

Also add a clean target, give absolute path to a2x, add a dependency
on the output file, and add to .PHONY.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Peter: build in docs/manual/<format>, fix clean targets]
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>