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.