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

h8liu/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,dsriram/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,h8liu/golang,dsriram/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,ssrl/go,apparentlymart/golang,tv42/old-go,tv42/old-go,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,h8liu/golang,pombredanne/go-deleteme,gnanderson/go,ssrl/go,pombredanne/go-deleteme,tv42/old-go,h8liu/golang,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,h8liu/golang,tv42/old-go,dsriram/golang,apparentlymart/golang,dsriram/golang,ssrl/go,h8liu/golang,ssrl/go,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,ssrl/go,ssrl/go,apparentlymart/golang,dsriram/golang,h8liu/golang,dsriram/golang,ssrl/go