dist: fix postinstall script for Darwin
The postinstall script causes the installation to fail because the last
step that copies files for Xcode is broken. Two details can cause the
command to fail:
1. The XCODE_MISC_DIR value has a space. Without quotes in the cp
command, cp will just complain that this is an invalid syntax.
2. The source of the cp is a directory with two subdirectories.
We actually want the files for either Xcode 3 or Xcode 4 to be copied.
Using xcodebuild -version, we check for the Xcode version and
select which of xcode/3/* or xcode/4/* should be the source
of the copy.
Fixes #5874.
R=golang-dev, minux.ma, adg
CC=golang-dev
https://golang.org/cl/
10893044