superenv: Improve compiler selection

This fixes superenv on Xcode 3.2.6, and also fixes C++ software with
gcc-4.2 on all Xcodes.

Library/ENV/4.3/cc

@@ -62,12 +62,15 @@ class Cmd
       # the work since Makefiles are dumb and include a lot of excess flags.
       ENV['HOMEBREW_CC']
     when 'c++'
-      if ENV['HOMEBREW_CC'] =~ /llvm-gcc/
-        'g++'
-      elsif ENV['HOMEBREW_CC'] =~ /gcc-4.2/
-        'g++-4.2'
-      else
+      case ENV['HOMEBREW_CC']
+      when /clang/
         'clang++'
+      when /llvm-gcc/
+        'g++'
+      when /gcc(-4.2)?$/
+        'g++-4.2'
+      when /gcc-4.0/
+        'g++-4.0'
       end
     else
       @arg0

Library/Homebrew/superenv.rb

@@ -93,7 +93,7 @@ class << ENV
       "gcc"
     elsif ENV['HOMEBREW_CC']
       case ENV['HOMEBREW_CC']
-        when 'clang', 'gcc' then ENV['HOMEBREW_CC']
+        when 'clang', 'gcc', 'gcc-4.0' then ENV['HOMEBREW_CC']
         when 'llvm', 'llvm-gcc' then 'llvm-gcc'
       else
         opoo "Invalid value for HOMEBREW_CC: #{ENV['HOMEBREW_CC']}"
@@ -103,7 +103,12 @@ class << ENV
       raise
     end
   rescue
-    "clang"
+    case MacOS.default_compiler
+    when :clang   then 'clang'
+    when :llvm    then 'llvm-gcc'
+    when :gcc     then 'gcc'
+    when :gcc_4_0 then 'gcc-4.0'
+    end
   end
 
   def determine_path