go/build: allow colon in #cgo flags

This makes it possible to say -I c:/foo on Windows.

Fixes #2683 comment #3.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5540043

src/pkg/go/build/dir.go

@@ -476,7 +476,7 @@ func (ctxt *Context) saveCgo(filename string, di *DirInfo, cg *ast.CommentGroup)
 	return nil
 }
 
-var safeBytes = []byte("+-.,/0123456789=ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz")
+var safeBytes = []byte("+-.,/0123456789=ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz:")
 
 func safeName(s string) bool {
 	if s == "" {