brew: use a shell wrapper script for brew

This reverts commit dfd4dd1.

Fixes the problems from Homebrew#17683 and Homebrew#17685.

Library/brew.rb

@@ -3,7 +3,8 @@
 
 std_trap = trap("INT") { exit! 130 } # no backtrace thanks
 
-HOMEBREW_BREW_FILE = ENV['HOMEBREW_BREW_FILE'] = File.expand_path(__FILE__)
+HOMEBREW_BREW_FILE = ENV['HOMEBREW_BREW_FILE']
+HOMEBREW_BREW_FILE = ENV['HOMEBREW_BREW_FILE'] ||= File.expand_path(__FILE__)
 
 if ARGV == %w{--prefix}
   puts File.dirname(File.dirname(HOMEBREW_BREW_FILE))

bin/brew

@@ -0,0 +1,24 @@
+#!/bin/sh
+BREW_FILE_DIRECTORY=$(dirname "$0")
+BREW_FILE_DIRECTORY=$(cd "$BREW_FILE_DIRECTORY" && pwd -P)
+
+BREW_FILENAME=$(basename "$0")
+export HOMEBREW_BREW_FILE="$BREW_FILE_DIRECTORY/$BREW_FILENAME"
+
+BREW_SYMLINK=$(readlink $0)
+if [ -n "$BREW_SYMLINK" ]
+then
+	BREW_SYMLINK_DIRECTORY=$(dirname "$BREW_SYMLINK")
+	BREW_FILE_DIRECTORY=$(cd "$BREW_FILE_DIRECTORY" &&
+	                      cd "$BREW_SYMLINK_DIRECTORY" && pwd -P)
+fi
+
+BREW_LIBRARY_DIRECTORY=$(cd "$BREW_FILE_DIRECTORY"/../Library && pwd -P)
+
+BREW_SYSTEM=$(uname -s | tr "[:upper:]" "[:lower:]")
+if [ "$BREW_SYSTEM" = "darwin" ]
+then
+	exec "$BREW_LIBRARY_DIRECTORY/brew.rb" "$@"
+else
+	exec ruby -W0 "$BREW_LIBRARY_DIRECTORY/brew.rb" "$@"
+fi