configure.ac: handle undefined LEVEL1_DCACHE_LINESIZE

On some platforms (riscv64, s390x) this value is "undefined" as returned
from getconf. We also need to handle this to avoid using the string
"undefined" blindly in further #defines.

configure.ac

@@ -2159,7 +2159,7 @@ fi
     AC_PATH_PROG(HAVE_GETCONF_CMD, getconf, "no")
     if test "$HAVE_GETCONF_CMD" != "no"; then
         CLS=$(getconf LEVEL1_DCACHE_LINESIZE)
-        if [test "$CLS" != "" && test "$CLS" != "0"]; then
+        if [test "$CLS" != "undefined" && test "$CLS" != "" && test "$CLS" != "0"]; then
             AC_DEFINE_UNQUOTED([CLS],[${CLS}],[L1 cache line size])
         else
             AC_DEFINE([CLS],[64],[L1 cache line size])