tools, build system: Detect distro-variant directories at build-time

tools, build system: Detect distro-variant directories at build-time

In b59f87f56b1e, the defaults for some of the directory config
variables implicitly changed.  In this commit we arrange for the
defaults to be auto-detected (with the same logic as the run-time
scripts use).

This will make the build more likely to produce answers which the
builder intends.  (And, in particular, it should un-break the
automated tests, which didn't set these variables explicitly and
therefore got broken by b59f87f56b1e.)

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>