configure: GSSAPI detection on ancient Linux distros

On some ancient distributions such as RHEL-3, <gssapi/gssapi_krb5.h> needs
to be processed after <gssapi/gssapi.h>, but does not include it itself.
This patch checks for <gssapi/gssapi.h> first and then includes it
in the test for <gssapi/gssapi_krb5.h>, resolving the problem.

Without the patch, <gssapi/gssapi_krb5.h> is "present but cannot be
compiled".