The current mechanism fails if clang cannot be executed by the current
user.  Using the `-x` operator for `test` return TRUE if the file is
executable by the user.

Change-Id: I0f3c8dc3880c1ce5a8a833ff3109eb96853184af
Reviewed-on: https://go-review.googlesource.com/16752Reviewed-by: Ian Lance Taylor <iant@golang.org>