Don't hang if xcode-select -print-path is "/"

Introducing MacOS.xctools_fucked?.

Refs Homebrew/homebrew#10293.

Library/Homebrew/cmd/--env.rb

@@ -18,7 +18,7 @@ module Homebrew extend self
       value = env[k]
       if value
         results = value
-        if value =~ /^[^\s]*xcrun (.*)/
+        if value =~ %r{/usr/bin/xcrun (.*)}
           path = `/usr/bin/xcrun -find #{$1}`
           results += " => #{path}"
         elsif File.exists? value and File.symlink? value

Library/Homebrew/extend/ENV.rb

@@ -94,7 +94,7 @@ module HomebrewEnvExtension
   def xcrun tool
     if File.executable? "/usr/bin/#{tool}"
       "/usr/bin/#{tool}"
-    elsif system "/usr/bin/xcrun -find #{tool} 2>1 1>/dev/null"
+    elsif not MacOS.xctools_fucked? and system "/usr/bin/xcrun -find #{tool} 2>1 1>/dev/null"
       # xcrun was provided first with Xcode 4.3 and allows us to proxy
       # tool usage thus avoiding various bugs
       "/usr/bin/xcrun #{tool}"

Library/Homebrew/utils.rb

@@ -266,9 +266,17 @@ module MacOS extend self
     end
   end
 
+  def xctools_fucked?
+    # Xcode 4.3 tools hang if "/" is set
+    `/usr/bin/xcode-select -print-path 2>/dev/null`.chomp == "/"
+  end
+
   def default_cc
-    cc = `/usr/bin/xcrun -find cc 2> /dev/null`.chomp
-    cc = "#{dev_tools_path}/cc" if cc.empty? or not $?.success?
+    cc = unless xctools_fucked?
+      out = `/usr/bin/xcrun -find cc 2> /dev/null`.chomp
+      out if $?.success?
+    end
+    cc = "#{dev_tools_path}/cc" if cc.nil? or cc.empty?
 
     unless File.executable? cc
       # If xcode-select isn't setup then xcrun fails and on Xcode 4.3