cmd/dist, build: support building statically linked toolchain

Added a new $GO_DISTFLAGS to make.bash, and while we're here,
added mention $CXX in make.bash (CL 13704044).

Fixes #6448.
Update #3564
We can pass GO_DISTFLAGS=-s from misc/dist to make.bash so that
it will build a statically linked toolchain.
(Note: OS X doesn't have the concept of static linking, so don't
pass GO_DISTFLAGS=-s for OS X builds)

R=adg, rsc, iant
CC=golang-dev
https://golang.org/cl/13887043

src/cmd/dist/a.h

@@ -112,6 +112,7 @@ void	goc2c(char*, char*);
 
 // main.c
 extern int vflag;
+extern int sflag;
 void	usage(void);
 void	xmain(int argc, char **argv);
 

src/cmd/dist/build.c

@@ -720,6 +720,8 @@ install(char *dir)
 			vadd(&link, bpathf(&b, "%s/%s", tooldir, name));
 		} else {
 			vcopy(&link, gccargs.p, gccargs.len);
+			if(sflag)
+				vadd(&link, "-static");
 			vadd(&link, "-o");
 			targ = link.len;
 			vadd(&link, bpathf(&b, "%s/%s%s", tooldir, name, exe));
@@ -1527,6 +1529,9 @@ cmdbootstrap(int argc, char **argv)
 	case 'a':
 		rebuildall = 1;
 		break;
+	case 's':
+		sflag++;
+		break;
 	case 'v':
 		vflag++;
 		break;
@@ -1613,6 +1618,9 @@ cmdinstall(int argc, char **argv)
 	int i;
 
 	ARGBEGIN{
+	case 's':
+		sflag++;
+		break;
 	case 'v':
 		vflag++;
 		break;

src/cmd/dist/main.c

@@ -5,6 +5,7 @@
 #include "a.h"
 
 int vflag;
+int sflag;
 char *argv0;
 
 // cmdtab records the available commands.

src/make.bash

@@ -37,6 +37,11 @@
 #
 # CC: Command line to run to get at host C compiler.
 # Default is "gcc". Also supported: "clang".
+# CXX: Command line to run to get at host C++ compiler, only recorded
+# for cgo use. Default is "g++". Also supported: "clang++".
+#
+# GO_DISTFLAGS: extra flags to provide to "dist bootstrap". Use "-s"
+# to build a statically linked toolchain.
 
 set -e
 if [ ! -f run.bash ]; then
@@ -140,7 +145,7 @@ buildall="-a"
 if [ "$1" = "--no-clean" ]; then
 	buildall=""
 fi
-./cmd/dist/dist bootstrap $buildall -v # builds go_bootstrap
+./cmd/dist/dist bootstrap $buildall $GO_DISTFLAGS -v # builds go_bootstrap
 # Delay move of dist tool to now, because bootstrap may clear tool directory.
 mv cmd/dist/dist "$GOTOOLDIR"/dist
 "$GOTOOLDIR"/go_bootstrap clean -i std