Introduce ENV.O2 and use that for the postgresql build

Also only set ARCHFLAGS on non-Core Duo machines... though it seemed to build
fine even with it, it doenst make much sense to me... these changes are kind
of up for debate but it "works for me" this way...

Closes Homebrew/homebrew#75

Library/Homebrew/brewkit.rb

@@ -128,6 +128,12 @@ module HomebrewEnvExtension
     remove_from_cflags '-O4'
     append_to_cflags '-O3'
   end
+  def O2
+    # Sometimes O3 doesn't work or produces bad binaries
+    remove_from_cflags '-O4'
+    remove_from_cflags '-O3'
+    append_to_cflags '-O2'
+  end
   def gcc_4_2
     # Sometimes you want to downgrade from LLVM to GCC 4.2
     self['CC']="gcc-4.2"