Comment 1 for bug 1097670

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]
    […]