* Enable -Werror for GCC/Clang builds
Fixes many of the errors this produces, and disables a handful that
didn't have solutions that were obvious (to me).
* Check for `-W*` flags empirically
* Remove cl_APPLE_fp64_basic_ops support
* Undo NAN conversion fix
* Add comments to warning override flags
* Remove unneeded STRINGIFY definition
* Fix tautological compare issue in basic
* Use ABS_ERROR macro in image tests
* Use fabs for ABS_ERROR macro
* Move ABS_ERROR definition to common header
