go/build: fix cgo ${SRCDIR} substitution when that variable contains spaces

go/build: fix cgo ${SRCDIR} substitution when that variable contains spaces

When the source directory path contains spaces, cgo directives
cannot be properly validated:

$ pwd
/root/src/issue 11868

$ cat main.go
package main
//#cgo CFLAGS: -I${SRCDIR}/../../include
import "C"
func main() {
}

$ go build
can't load package: package issue 11868: /root/src/issue 11868/main.go:
 malformed #cgo argument: -I/root/src/issue 11868/../../include

Make sure spaces are tolerated in ${SRCDIR} when this variable
is expanded. This applies to ${SRCDIR} only. Shell safety
checks are still done in the same exact way for anything else.

Fixes #11868

Change-Id: I93d1d2b5ab167caa7ae353fe46fb8f69f1f06969
Reviewed-on: https://go-review.googlesource.com/16302
Reviewed-by: Russ Cox <rsc@golang.org>