gc: enable building under clang/2.9
gc: enable building under clang/2.9

To build under clang, pass the path to clang in CC when
calling ./make.bash

CC=/opt/llvm/llvm-2.9/bin/clang ./make.bash

Credit goes to jmhodges for suggestions.

R=jeff, r, ality
CC=golang-dev
http://codereview.appspot.com/4527098

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go