Neuter gratuitous OS check that is not used for anything at all.

--- configure.ac.orig	2020-05-26 20:29:47 UTC
+++ configure.ac
@@ -294,7 +294,7 @@ case "${host_os}" in
         build_mac=yes
         ;;
     *)
-        AC_MSG_ERROR(["OS $host_os is not supported"])
+        AC_MSG_NOTICE([I'm a teapot])
         ;;
 esac