...replacing hard-coded GCC version checks.  Those checks that guard

  #pragma GCC diagnostic ignored "-Wnon-virtual-dtor"

appear relevant only for GCC itself, not Clang (which used to fail the old
guards because it typically announces itself with a rather low
__GNUC__/__GNUC_MINOR__ version), see 6e67c03d
"Enable -Wnon-virtual-dtor for GCC 4.6"

Change-Id: I6bfa4d5caa6192e7a203ce829682bf6bb8d61a1b