BIND 9.16 triggers build warnings on FreeBSD 11.4

With Clang 10.0.0 on FreeBSD 11.4, compiling lib/dns/spnego.c triggers the following warnings:

spnego.c:361:11: error: converting the result of '<<' to a boolean always evaluates to true [-Werror,-Wtautological-constant-compare]
                return (GSS_S_DEFECTIVE_TOKEN);
                        ^
/usr/include/gssapi/gssapi.h:423:41: note: expanded from macro 'GSS_S_DEFECTIVE_TOKEN'
#define GSS_S_DEFECTIVE_TOKEN      (9ul << GSS_C_ROUTINE_ERROR_OFFSET)
                                        ^
spnego.c:366:11: error: converting the result of '<<' to a boolean always evaluates to true [-Werror,-Wtautological-constant-compare]
                return (GSS_S_DEFECTIVE_TOKEN);
                        ^
/usr/include/gssapi/gssapi.h:423:41: note: expanded from macro 'GSS_S_DEFECTIVE_TOKEN'
#define GSS_S_DEFECTIVE_TOKEN      (9ul << GSS_C_ROUTINE_ERROR_OFFSET)
                                        ^
spnego.c:371:12: error: converting the result of '<<' to a boolean always evaluates to true [-Werror,-Wtautological-constant-compare]
                        return (GSS_S_DEFECTIVE_TOKEN);
                                ^
/usr/include/gssapi/gssapi.h:423:41: note: expanded from macro 'GSS_S_DEFECTIVE_TOKEN'
#define GSS_S_DEFECTIVE_TOKEN      (9ul << GSS_C_ROUTINE_ERROR_OFFSET)
                                        ^
spnego.c:376:11: error: converting the result of '<<' to a boolean always evaluates to true [-Werror,-Wtautological-constant-compare]
                return (GSS_S_DEFECTIVE_TOKEN);
                        ^
/usr/include/gssapi/gssapi.h:423:41: note: expanded from macro 'GSS_S_DEFECTIVE_TOKEN'
#define GSS_S_DEFECTIVE_TOKEN      (9ul << GSS_C_ROUTINE_ERROR_OFFSET)
                                        ^
spnego.c:380:11: error: converting the result of '<<' to a boolean always evaluates to true [-Werror,-Wtautological-constant-compare]
                return (GSS_S_DEFECTIVE_TOKEN);
                        ^
/usr/include/gssapi/gssapi.h:423:41: note: expanded from macro 'GSS_S_DEFECTIVE_TOKEN'
#define GSS_S_DEFECTIVE_TOKEN      (9ul << GSS_C_ROUTINE_ERROR_OFFSET)
                                        ^
5 errors generated.

For some reason, the buster build (which uses Clang 10.0.1) is happy with this code as it is 🤷

The prototype of lib/dns/spnego.c:cmp_gss_type() was changed back in b105ccee. v9_11 is not affected because !546 (merged) was not backported. main is not affected, either, because 978c7b2e dropped lib/dns/spnego.c altogether.