AFAICT, gcc stopped treating unknown "-Wno" flags as unconditional errors starting
with gcc-4.4, and the kernel's min supported version is 5.1. gcc-4.4 through
gcc-9.5 all print a mild warning if there's a different error, but otherwise
silently ignore the uknown "-Wno".
cc1: warning: unrecognized command line option '-Wno-gnu-variable-sized-type-not-at-end'