Xcode is still installed, even if it's as old as 3.0

Instead tell them to upgrade.

Problem was Xcode 3.0 didn't come with GCC 4.2.

Library/Homebrew/brew.h.rb

@@ -465,6 +465,12 @@ def gcc_build
   `/usr/bin/gcc-4.2 -v 2>&1` =~ /build (\d{4,})/
   if $1
     $1.to_i 
+  elsif system "/usr/bin/which gcc"
+    # Xcode 3.0 didn't come with gcc-4.2
+    # We can't change the above regex to use gcc because the version numbers
+    # are different and thus, not useful.
+    # FIXME I bet you 20 quid this causes a side effect — magic values tend to
+    401
   else
     nil
   end