当没有xcode时，我们使用homebrew会提示出错。
Error: Failure while executing: /usr/bin/otool -L /usr/bin/install_name_tool
或者出现
Error: The /usr/local directory is not writable.
Even if this directory was writable when you installed Homebrew, other
software may change permissions on this directory. Some versions of the
"InstantOn" component of Airfoil are known to do this.
You should probably change the ownership and permissions of /usr/local
back to your user account.
sudo chown -R $(whoami):admin /usr/local
在某些情况下，我们需要使用mac的命令行工具（如homebrew），但不想安装xcode开发环境。这时，我们需要
更新xcode的开发链接路径，使之指向实际的命令行工具路径。
sudo xcode-select --switch /Library/Developer/CommandLineTools
这样，homebrew就能找到环境了。