Miscellaneous gnu backend improvements

The main change is to allow absolute paths as build directories. Notice that it's unrelated to the rest of the discussion -- whatever anchors we use, `_builddir` would still need to be set like this.

`$_` thing cost me quite a bit of time, mostly because I couldn't understand why did this suddenly stop working under Debian (where I tested it when I had implemented this initially) while continuing to work under Cygwin. Turns out I changed `/bin/sh` from bash to dash in the meanwhile (Shellshock...) and dash has yet another bug here.