Define HAVE_GSSMIT if <gssapi/{gssapi.h,gssapi_generic.h,gssapi_krb5.h}> are
available, otherwise define HAVE_GSSHEIMDAL if <gssapi.h> is available.
Only define GSS_C_NT_HOSTBASED_SERVICE to gss_nt_service_name if
GSS_C_NT_HOSTBASED_SERVICE isn't declared by the gssapi headers. This should
avoid breakage in case we wrongly recognize Heimdal as MIT again.