Revert "Simplify the checks for supported compiler warning flags"

Authored by flherne on Apr 14 2017, 2:44 PM.

Description

Revert "Simplify the checks for supported compiler warning flags"

Doesn't compile on either GCC 5.2.1 (the CI) or 6.3.1 (my laptop).

This reverts commit 26918c8c3afe21e801d00774c2bf7ee3e2bc5ecd.