goinstall: handle .c files with gc when cgo isn't used
goinstall: handle .c files with gc when cgo isn't used

As a data point, this enables goinstall to handle the standard
syscall package almost unchanged (there's one file with the _bsd
extension, and a .c file which isn't supposed to be compiled in).

R=rsc
CC=golang-dev
http://codereview.appspot.com/4259057

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go