OpenBSD don't support TLS

under some compiler (gcc 4.8.4 in particular), the auto-detection of TLS
don't work properly.

force tls to be disabled.

the testsuite pass under gcc (4.8.4) and gcc (4.2.1)
diff --git a/configure.ac b/configure.ac
index be49743..bf2ac3a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -283,7 +283,13 @@
 	abi="elf"
 	AC_DEFINE([JEMALLOC_PURGE_MADVISE_FREE], [ ])
 	;;
-  *-*-openbsd*|*-*-bitrig*)
+  *-*-openbsd*)
+	CFLAGS="$CFLAGS"
+	abi="elf"
+	AC_DEFINE([JEMALLOC_PURGE_MADVISE_FREE], [ ])
+	force_tls="0"
+	;;
+  *-*-bitrig*)
 	CFLAGS="$CFLAGS"
 	abi="elf"
 	AC_DEFINE([JEMALLOC_PURGE_MADVISE_FREE], [ ])