fixed chicken build on older OS X

Chicken 4.9.0.1 attempts to use XCode's newer directory structure while building on OS X, which causes builds on Snow Leopard (with its older XCode and directory structure) to fail. 
Fixed it by modifying two makefile variables (XCODE_DEVELOPER and XCODE_TOOL_PATH) when MacOS.version is SL or earlier.

The Chicken developers have been notified of this, as well: https://bugs.call-cc.org/ticket/1140
Rather than hardcoding, can do:

```ruby
if MacOS::Xcode.installed?
  args << "XCODE_DEVELOPER=#{MacOS::Xcode.prefix}"
  args << "XCODE_TOOL_PATH=#{MacOS.sdk_path}/usr/bin"
end
```