If TESLA_BUILD_DIR isn't set, try to find tesla-<tool> in the directory
the wrapper script is in.  This fixes build problems when the script is
not found in the current path.

The use of which to find the path of the wrapper is somewhat bogus, but
seems to be the best we can do since neither realpath(1) or readlink -f
are portable.
Merge branch 'master' of https://github.com/CTSRD-TESLA/TESLA