brew: error out if bad xcode-select path given

Since a bad xcode-select path can cause many brew commands to freeze,
it's worth warning users and bailing out as early as possible.

The doctor check wasn't good enough, since the doctor could freeze
before the warning about this was ever printed.

Library/Homebrew/cmd/doctor.rb

@@ -437,15 +437,7 @@ def check_xcode_prefix_exists
 end
 
 def check_xcode_select_path
-  # with the advent of CLT-only support, we don't need xcode-select
+  if not MacOS::CLT.installed? and not File.file? "#{MacOS::Xcode.folder}/usr/bin/xcodebuild"
-
-  if MacOS::Xcode.bad_xcode_select_path?
-    <<-EOS.undent
-      Your xcode-select path is set to /
-      You must unset it or builds will hang:
-        sudo rm /usr/share/xcode-select/xcode_dir_*
-    EOS
-  elsif not MacOS::CLT.installed? and not File.file? "#{MacOS::Xcode.folder}/usr/bin/xcodebuild"
     path = MacOS.app_with_bundle_id(MacOS::Xcode::V4_BUNDLE_ID) || MacOS.app_with_bundle_id(MacOS::Xcode::V3_BUNDLE_ID)
     path = '/Developer' if path.nil? or not path.directory?
     <<-EOS.undent

Library/brew.rb

@@ -16,6 +16,22 @@ $:.unshift(HOMEBREW_LIBRARY_PATH + '/vendor')
 $:.unshift(HOMEBREW_LIBRARY_PATH)
 require 'global'
 
+# Check for bad xcode-select before anything else, because `doctor` and
+# many other things will hang
+# Note that this bug was fixed in 10.9
+if `xcode-select -print-path 2>/dev/null`.chomp == '/' && MacOS.version < :mavericks
+  ofail <<-EOS.undent
+  Your xcode-select path is currently set to '/'.
+  This causes the `xcrun` tool to hang, and can render Homebrew unusable.
+  If you are using Xcode, you should:
+    sudo xcode-select -switch /Applications/Xcode.app
+  Otherwise, you should:
+    sudo rm -rf /usr/share/xcode-select
+  EOS
+
+  exit -1
+end
+
 case ARGV.first when '-h', '--help', '--usage', '-?', 'help', nil
   require 'cmd/help'
   puts Homebrew.help_s