Tweak to avoid building Quicklisp on GCL
avoid dirname tests on sbcl also
Adding reference to books/std/std-customization.lsp from :doc topic working-with-packages.
Fixing up the HTTP_PROXY_WITH_PORT reference.
Adding a pointer to the explanation for how to install quicklisp from behind a proxy
Simplified the instructions for installing quicklisp behind a proxy.  Thanks to Kaufmann for helping me prune these instructions.