Bug 1385380 - Detect watchman more resiliently; r=glandium

Before, a non-runnable watchman would result in configure
error.

A trivial refactor to ignore `watchman version` errors would still
result in setting WATCHMAN and exposing its presence to downstream
consumers.

Since we want WATCHMAN tied to a working watchman install, we
refactor the code so that binary location and its version test
are in the same function. They are either both defined or none
of them are.

MozReview-Commit-ID: 7wvBvYuOlmJ

--HG--
extra : rebase_source : d9cc648cdb8253bf8e413ec0fa5e969aa68f75b9