Clang does not know about that that switch [1] and reports a warning.
$ CC=clang ./waf configure
$ ./waf build
[…]
clang: warning: unknown warning option '-Wno-unused-but-set-variable'; did you mean '-Wno-unused-variable'?
warning: unknown warning option '-Wno-unused-but-set-variable'; did you mean '-Wno-unused-variable'? [-Wunknown-warning-option]
[…]
Clang does not know about that that switch [1] and reports a warning.
$ CC=clang ./waf configure but-set- variable' ; did you mean '-Wno-unused- variable' ? but-set- variable' ; did you mean '-Wno-unused- variable' ? [-Wunknown- warning- option]
$ ./waf build
[…]
clang: warning: unknown warning option '-Wno-unused-
warning: unknown warning option '-Wno-unused-
[…]