Improve the OS X llvm-gcc error

  local gcc="$(locate_gcc || true)"

  if [ -z "$gcc" ]; then

    { echo

      echo "ERROR: This package must be compiled with GCC, and we"

      echo "couldn't find a suitable \`gcc' binary on your system."

      echo "Please install GCC and try again."

      echo "ERROR: This package must be compiled with GCC, but ruby-build couldn't"

      echo "find a suitable \`gcc\` executable on your system. Please install GCC"

      echo "and try again."

      echo

      if [ "$(uname -s)" = "Darwin" ]; then

        echo "As of version 4.2, Xcode is LLVM-only and no longer"

        echo "includes GCC. You can install GCC with these binary"

        echo "packages on Mac OS X:"

        local esc=$'\033'

        echo "Apple no longer includes the official GCC compiler with Xcode as of"

        echo "version 4.2. Instead, the \`gcc\` executable is a symlink to \`llvm-gcc\`,"

        echo "a modified version of GCC which outputs LLVM bytecode."

        echo

        echo "https://github.com/kennethreitz/osx-gcc-installer/downloads"

        echo "For most programs the \`llvm-gcc\` compiler works fine. However,"

        echo "versions of Ruby older than 1.9.3-p125 are incompatible with"

        echo "\`llvm-gcc\`. To build older versions of Ruby you must have the official"

        echo "GCC compiler installed on your system."

        echo

        echo "You can install the official GCC compiler on OS X using these binary"

        echo "packages: ${esc}[4mhttps://github.com/kennethreitz/osx-gcc-installer/downloads${esc}[0m"

        echo

        echo "You will need to install the official GCC compiler to build older"

        echo "versions of Ruby even if you have installed Apple's Command Line Tools"

        echo "for Xcode package. The Command Line Tools for Xcode package only"

        echo "includes \`llvm-gcc\`."

      fi

    } >&3

    return 1