build: exclude packages that fail on Plan 9 (for now)

All but two packages depend on net:
        debug/proc
        os/signal

With this change, we can produce
a working build with GOOS=plan9.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/4639053

src/pkg/Makefile

@@ -182,6 +182,36 @@ DIRS+=\
 
 endif
 
+ifeq ($(GOOS),plan9)
+NOPLAN9BUILD=\
+	crypto/tls\
+	debug/proc\
+	exp/gui/x11\
+	expvar\
+	http\
+	http/cgi\
+	http/fcgi\
+	http/httptest\
+	http/pprof\
+	http/spdy\
+	mail\
+	mime/multipart\
+	net\
+	net/dict\
+	net/textproto\
+	netchan\
+	os/signal\
+	rpc\
+	rpc/jsonrpc\
+	smtp\
+	syslog\
+	websocket\
+	../cmd/godoc\
+	../cmd/goinstall\
+
+DIRS:=$(filter-out $(NOPLAN9BUILD),$(DIRS))
+endif
+
 NOTEST+=\
 	crypto\
 	crypto/openpgp/error\

src/pkg/runtime/cgo/Makefile

@@ -10,6 +10,10 @@ ifeq ($(GOARCH),arm)
 ENABLED:=0
 endif
 
+ifeq ($(GOOS),plan9)
+ENABLED:=0
+endif
+
 ifeq ($(DISABLE_CGO),1)
 ENABLED:=0
 endif