Fix p6doc build errors

[Kaiepi]

The problem

zef install p6doc fails after building.

Solution provided

After looking through zef's code, I found that it considers the build to have passed only if a truthy value is returned, so I made the build method return True.

[JJ]

Um, not sure about that. If it does not build, it might be because there's some error somewhere. Always returning True might not be a good idea. It will kick the error down the road, and will eventually show up someplace else...

[Kaiepi]

Would checking if all the files were copied over and returning True/False depending on whether or not it succeeded be a better idea?

[Kaiepi]

Something like this

[zoffixznet]

Can this be merged now?

[coke]

Looks good to me, addresses JJ's concern.