cmd/cgo: don't depend on runtime/cgo if !CgoEnabled

Fixes the build when CGO_ENABLED=0.

Change-Id: I7f3c67d61e156e69536558fda0a0a4b429b82bbd
Reviewed-on: https://go-review.googlesource.com/33104
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/pkg.go

@@ -913,6 +913,7 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 		importPaths = append(importPaths, "syscall")
 	}
 
+	if buildContext.CgoEnabled && p.Name == "main" && !p.Goroot {
 		// Currently build modes c-shared, pie (on systems that do not
 		// support PIE with internal linking mode), plugin, and
 		// -linkshared force external linking mode, as of course does
@@ -928,9 +929,10 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 				linkmodeExternal = true
 			}
 		}
-	if p.Name == "main" && !p.Goroot && (buildBuildmode == "c-shared" || buildBuildmode == "plugin" || pieCgo || buildLinkshared || linkmodeExternal) {
+		if buildBuildmode == "c-shared" || buildBuildmode == "plugin" || pieCgo || buildLinkshared || linkmodeExternal {
 			importPaths = append(importPaths, "runtime/cgo")
 		}
+	}
 
 	// Everything depends on runtime, except runtime, its internal
 	// subpackages, and unsafe.