More thorough sanitisation of clang flags
    
    Turns out that the AFL "build system" (I use the term loosely) doesn't
    fail on all errors, but instead decides to silently compile without
    clang support.  We weren't properly sanitising the clang flags so this
    caused the build to degrade silently.
    
    Updates: commit b5bc344b308a08921049080d42191acaf1f10122