cmd/go: quote fragments in CGO_ env variables reported by 'go env'

These fields have been parsed as quoted fields since CL 334732,
but we missed the unparsing side in 'go env'.

Certain scripts (notably make.ba{sh,t}) expect to be able to set the
environment to exactly what 'go env' reports, so for round-trip
purposes it is important to match the marshaling and unmarshaling
functions.

(Noticed while debugging #52009.)
Updates #41400

Change-Id: I0ff39b7a6e1328111c285c97cd23f79b723f3c73
Reviewed-on: https://go-review.googlesource.com/c/go/+/398058
Trust: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: Bryan C. Mills

src/cmd/go/internal/envcmd/env.go

@@ -184,15 +184,23 @@ func ExtraEnvVarsCostly() []cfg.EnvVar {
 	}
 	cmd := b.GccCmd(".", "")
 
+	join := func(s []string) string {
+		q, err := quoted.Join(s)
+		if err != nil {
+			return strings.Join(s, " ")
+		}
+		return q
+	}
+
 	return []cfg.EnvVar{
 		// Note: Update the switch in runEnv below when adding to this list.
-		{Name: "CGO_CFLAGS", Value: strings.Join(cflags, " ")},
-		{Name: "CGO_CPPFLAGS", Value: strings.Join(cppflags, " ")},
-		{Name: "CGO_CXXFLAGS", Value: strings.Join(cxxflags, " ")},
-		{Name: "CGO_FFLAGS", Value: strings.Join(fflags, " ")},
-		{Name: "CGO_LDFLAGS", Value: strings.Join(ldflags, " ")},
+		{Name: "CGO_CFLAGS", Value: join(cflags)},
+		{Name: "CGO_CPPFLAGS", Value: join(cppflags)},
+		{Name: "CGO_CXXFLAGS", Value: join(cxxflags)},
+		{Name: "CGO_FFLAGS", Value: join(fflags)},
+		{Name: "CGO_LDFLAGS", Value: join(ldflags)},
 		{Name: "PKG_CONFIG", Value: b.PkgconfigCmd()},
-		{Name: "GOGCCFLAGS", Value: strings.Join(cmd[3:], " ")},
+		{Name: "GOGCCFLAGS", Value: join(cmd[3:])},
 	}
 }