Fix threaded initialization and enable it on Linux.

Reported by Mike Hommey.
diff --git a/configure.ac b/configure.ac
index 8e6a309..16f0378 100644
--- a/configure.ac
+++ b/configure.ac
@@ -228,6 +228,7 @@
 	CPPFLAGS="$CPPFLAGS -D_GNU_SOURCE"
 	abi="elf"
 	AC_DEFINE([JEMALLOC_PURGE_MADVISE_DONTNEED], [ ])
+	AC_DEFINE([JEMALLOC_THREADED_INIT], [ ])
 	RPATH="-Wl,-rpath,"
 	;;
   *-*-netbsd*)