cmd/gc: make sure use of pthread for gcc-4.5 and beyond
cmd/gc: make sure use of pthread for gcc-4.5 and beyond

R=golang-dev, rsc, n13m3y3r, rogpeppe
CC=golang-dev
http://codereview.appspot.com/5501060
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go