"tkey-gssapi-credential" is broken in BIND 9.16.14 and 9.11.30
!4855 (merged) & !4857 (merged) left a bit of code in internal GSSAPI-related BIND headers
which causes the GSS_SPNEGO_MECHANISM preprocessor macro to be defined
to NULL if it is not defined by the headers provided by the
Kerberos/GSSAPI library used. This ultimately leads to the
dst_gssapi_acquirecred() function always failing, preventing any
named instance using tkey-gssapi-credential from working.
This is a regression since BIND 9.16.13 & 9.11.29.