cgo: fix build: use $GCC, not $CC
cgo: fix build: use $GCC, not $CC

I'm not sure there's a better way.
Cgo is juggling two different C compilers.

Fixes issue 1008.

R=iant
CC=golang-dev
http://codereview.appspot.com/1972044
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go