cmd/cgo: pass -Wsystem-headers when looking for errors

This works around a bug in GCC 4.8.0.

Fixes issue 5118.

R=golang-dev, r, minux.ma
CC=golang-dev
https://codereview.appspot.com/9120045
doc: pull front page featured articles using new blog JSON feed

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/9288045
spec: fix incorrect example

Fixes issue 5430.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/9305043
runtime: fix crash in select
runtime.park() can access freed select descriptor
due to a racing free in another thread.
See the comment for details.

Slightly modified version of dvyukov's CL 9259045.

No test yet.  Before this CL, the test described in issue 5422
would fail about every 40 times for me.  With this CL, I ran
the test 5900 times with no failures.

Fixes issue 5422.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/9311043
tag go1.1rc3

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/9124045