- use -fstack-protector only if both compiler and linker support it
- use -Wno-missing-field-initializers vs -Wno-extra depending on compiler support
- -Wno-pointer-sign only if compiler supports it
- add -Wno-address to avoid "the address of ‘buf’ will never be NULL" warning in
  cache_backend_cfg.c (and in other places where REPLACE() is used with local
  buf variable)