build: avoid bad environment interactions
build: avoid bad environment interactions

Specifically, $LIB is set on Windows.

Also diagnose attempt to build GOOS=darwin GOARCH=arm.
Otherwise the build fails mysteriously in package runtime.

Fixes issue 1065.

R=r
CC=golang-dev
http://codereview.appspot.com/2162043
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go