misc/cgo/test: test of issue 4339

This is not quite what that issue reports,
because this does not involve a DLL.
But I wanted to make sure this much was working.

Update issue 4339

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/13653043
cmd/cgo: don't say "gcc produced no output" if we ran clang

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/13420048
cmd/go: use pattern to prune file tree walk

For example, if the pattern is m... there is
no need to look in directories not beginning with m.

Fixes issue 5214.

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/13253049