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".

configure.ac

@@ -1137,10 +1137,17 @@ if test x"$want_gss" = xyes; then
     ],
     [
       dnl not found, check Heimdal or MIT
+      AC_CHECK_HEADERS([gssapi/gssapi.h], [], [not_mit=1])
       AC_CHECK_HEADERS(
-        [gssapi/gssapi.h gssapi/gssapi_generic.h gssapi/gssapi_krb5.h],
+        [gssapi/gssapi_generic.h gssapi/gssapi_krb5.h],
         [],
-        [not_mit=1])
+        [not_mit=1],
+        [
+AC_INCLUDES_DEFAULT
+#ifdef HAVE_GSSAPI_GSSAPI_H
+#include <gssapi/gssapi.h>
+#endif
+        ])
       if test "x$not_mit" = "x1"; then
         dnl MIT not found, check for Heimdal
         AC_CHECK_HEADER(gssapi.h,