Fix basename prototype collision string.h vs libgen.h

Fix basename prototype collision string.h vs libgen.h

        * libc/include/libgen.h (basename): Drop defining _BASENAME_DEFINED.
        Always define macro basename.  Add comment to explain why.
        * libc/include/string.h (basename): Check for basename instead of
        _BASENAME_DEFINED.  Drop __GNUC__ branch, always use basename macro.
        Change comment to explain why.  Add nonnull function attribute.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>