- Fix the MIT / Heimdal check for good:

  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.