cmd/dist, cmd/go: embed default C++ compiler into cmd/go

Fixes #6426.

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/13704044

src/cmd/dist/a.h

@@ -75,6 +75,7 @@ extern char *goroot_final;
 extern char *goextlinkenabled;
 extern char *goversion;
 extern char *defaultcc;
+extern char *defaultcxx;
 extern char *workdir;
 extern char *tooldir;
 extern char *slash;

src/cmd/dist/build.c

@@ -27,6 +27,7 @@ char *gochar;
 char *goversion;
 char *slash;	// / for unix, \ for windows
 char *defaultcc;
+char *defaultcxx;
 bool	rebuildall;
 bool defaultclang;
 
@@ -163,6 +164,15 @@ init(void)
 	}
 	defaultcc = btake(&b);
 
+	xgetenv(&b, "CXX");
+	if(b.len == 0) {
+		if(defaultclang)
+			bprintf(&b, "clang++");
+		else
+			bprintf(&b, "g++");
+	}
+	defaultcxx = btake(&b);
+
 	xsetenv("GOROOT", goroot);
 	xsetenv("GOARCH", goarch);
 	xsetenv("GOOS", goos);

src/cmd/dist/buildgo.c

@@ -12,6 +12,7 @@
 //
 //	package main
 //	const defaultCC = <defaultcc>
+//	const defaultCXX = <defaultcxx>
 //
 // It is invoked to write cmd/go/zdefaultcc.go
 // but we also write cmd/cgo/zdefaultcc.go.
@@ -28,8 +29,9 @@ mkzdefaultcc(char *dir, char *file)
 		"\n"
 		"package main\n"
 		"\n"
-		"const defaultCC = `%s`\n",
-		defaultcc);
+		"const defaultCC = `%s`\n"
+		"const defaultCXX = `%s`\n",
+		defaultcc, defaultcxx);
 
 	writefile(&out, file, 0);
 

src/cmd/go/build.go

@@ -1837,8 +1837,9 @@ func (b *builder) gccCmd(objdir string) []string {
 }
 
 // gxxCmd returns a g++ command line prefix
+// defaultCXX is defined in zdefaultcc.go, written by cmd/dist.
 func (b *builder) gxxCmd(objdir string) []string {
-	return b.ccompilerCmd("CXX", "g++", objdir)
+	return b.ccompilerCmd("CXX", defaultCXX, objdir)
 }
 
 // ccompilerCmd returns a command line prefix for the given environment

src/cmd/go/env.go

@@ -54,6 +54,8 @@ func mkEnv() []envVar {
 		cmd := b.gccCmd(".")
 		env = append(env, envVar{"CC", cmd[0]})
 		env = append(env, envVar{"GOGCCFLAGS", strings.Join(cmd[3:], " ")})
+		cmd = b.gxxCmd(".")
+		env = append(env, envVar{"CXX", cmd[0]})
 	}
 
 	if buildContext.CgoEnabled {