Ubuntu-12/14 has two packages which provide a binary file `ninja`:
1. ninja : http://forkbomb.org/ninja/
2. ninja-build : http://martine.github.io/ninja/
We had a case where a user has ninja installed instead of ninja-build,
then linja confuses that it has ninja-build and tries to use it.
This commit excludes a directory whose name includes "sbin" in finding a
system program. As a result, `which` will not find ninja[1] which
resides at /usr/sbin.
