cmSystemTools: Remove unreachable return on HP compiler

Follow up commit 1de08685 (cmSystemTools: Restore unreachable return to
silence warnings, 2014-04-02) by excluding the unreachable return on the
HP compiler as well as Clang.  Both recognize that it is unreachable and
warn.
Merge topic 'suppress-warning-in-comment' into next

a9fe65c3 cmSystemTools: Remove unreachable return on HP compiler