Clang by default compiles with -Winconsistent-missing-override, which
warns when a class declares virtual functions that override those in the
base class, and some but not all of them are explicitly declared
`override`.
GCC doesn't support this option, but has a stronger version,
-Wsuggest-override. In combination with -Werror, this means that any
virtual function that overrides another *must* be explicitly declared as
`override`.
This commit enables -Wsuggest-override where available. This means that
GCC users can't break the Clang build with inconsistent overrides (see
#1113 and #1114) and consequently that any build that passes the pull
request CI build on Jenkins won't break because of inconsistent
overrides.
tomangelo