Unexpected dependency on shell REVBIN variable.

[kfogel]

I think as of commit 8463ab1 we've been depending on the shell having a REVBIN variable in it. I agree, by the way, that the version of the Makefile prior to that commit had a buggy line for assigning REVBIN -- what was there wasn't always going to work either. But the new line assumes that REVBIN is set in the surrounding shell, which at least for me is not the case. If we were going to have that variable, we would probably want to call it OTS_REVBIN or something like that anyway (similarly to OTSDIR and OTS_DOCTOOLS_DIR).

It's not a showstopper currently because it only matters for .draft.pdf outputs -- that is, outputs with the "DRAFT" watermark.