disable fstack usage

disable fstack usage

It's only workable on gcc 4.6+... and in fact it's only
going to be interesting typically to the maintainer, so it's
commented out in Makefile.am.

Currently the worst remaining ones for x86_64 are

libwebsockets.c:2250:6:_lws_log	496	static
libwebsockets.c:2203:13:lwsl_emit_stderr	352	static
client.c:700:1:libwebsockets_generate_client_handshake	320	static
server.c:42:1:libwebsockets_decode_ssl_error	288	static
server-handshake.c:31:1:handshake_0405	272	static
client.c:368:1:lws_client_interpret_server_handshake	272	static
libwebsockets.c:447:1:libwebsockets_get_peer_addresses	256	static
daemonize.c:93:1:lws_daemonize	240	static
output.c:536:5:libwebsockets_serve_http_file	224	static
sha-1.c:131:1:sha1_step	192	static
sha-1.c:316:1:SHA1	160	static
libwebsockets.c:1604:1:libwebsocket_create_context	160	static

The top two there are error assembly buffers, they could be made
static but then they won't be usable from threaded user code.

Reported-by: Anders Brander <anders@brander.dk>
Signed-off-by: Andy Green <andy.green@linaro.org>