build: fix build with autoconf 2.59

Commit 350583c8 broke development on a RHEL 5 box, where the
ancient Autoconf 2.59 lacks AS_VERSION_STRING.  Rather than
backport the complex awk script that newer autoconf uses for
true strverscmp comparisons from the shell, it was easier to
just open-code a shell case statement.

* configure.ac (qemu_version): Open-code a replacement for
AS_VERSION_CHECK.

configure.ac

@@ -1121,8 +1121,11 @@ if test "$with_qemu:$with_yajl" = yes:check; then
     else
       [qemu_version_sed='s/.*ersion \([0-9.,]*\).*/\1/']
       qemu_version=`$QEMU -version | sed "$qemu_version_sed"`
-      AS_VERSION_COMPARE([$qemu_version], [0.15],
-                         [], [with_yajl=yes], [with_yajl=yes])
+      case $qemu_version in
+        [[1-9]].* | 0.15.* ) with_yajl=yes ;;
+        0.* | '' ) ;;
+        *) AC_MSG_ERROR([Unexpected qemu version string]) ;;
+      esac
     fi
   fi
 fi