cmd/dist, make.bash: pass -mmacosx-version-min=10.6 to gcc on Darwin

Fixes #5261.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/8798044

src/cmd/dist/build.c

@@ -622,6 +622,10 @@ install(char *dir)
 			// clang is too smart about unused command-line arguments
 			vadd(&gccargs, "-Qunused-arguments");
 		}
+		if(streq(gohostos, "darwin")) {
+			// golang.org/issue/5261
+			vadd(&gccargs, "-mmacosx-version-min=10.6");
+		}
 	}
 
 	islib = hasprefix(dir, "lib") || streq(dir, "cmd/cc") || streq(dir, "cmd/gc");

src/make.bash

@@ -111,6 +111,10 @@ case "$GOHOSTARCH" in
 386) mflag=-m32;;
 amd64) mflag=-m64;;
 esac
+if [ "$(uname)" == "Darwin" ]; then
+	# golang.org/issue/5261
+	mflag="$mflag -mmacosx-version-min=10.6"
+fi
 ${CC:-gcc} $mflag -O2 -Wall -Werror -o cmd/dist/dist -Icmd/dist "$DEFGOROOT" cmd/dist/*.c
 
 eval $(./cmd/dist/dist env -p)