-AC_ARG_WITH(sgen, [ --with-sgen=yes,no Extra Generational GC, default=yes],[buildsgen=$with_sgen],[buildsgen=yes])
+AC_ARG_WITH(sgen, [ --with-sgen=yes,no Extra Generational GC, default=yes],[buildsgen=$with_sgen],[buildsgen=yes])
+AC_ARG_WITH(sgen-default-concurrent, [ --with-sgen-default-concurrent=yes,no Use Concurrent GC, default=no],[],[with_sgen_default_concurrent=no])