superenv: --use-gcc should specify gcc-4.2

Since 'gcc' is a symlink to 'llvm-gcc' on Xcode 4.3+, --use-gcc and
--use-llvm were doing exactly the same thing. Combined with the
previous commit, this allows users with either a leftover
/usr/bin/gcc-4.2 or a homebrewed apple-gcc42 to build with gcc.

This doesn't however fix Xcode-only systems with apple-gcc42.

Fixes Homebrew/homebrew#17243.

Library/ENV/4.3/cc

@@ -60,8 +60,10 @@ class Cmd
       # the work since Makefiles are dumb and include a lot of excess flags.
       ENV['HOMEBREW_CC']
     when 'c++'
-      if ENV['HOMEBREW_CC'] =~ /gcc/
+      if ENV['HOMEBREW_CC'] =~ /llvm-gcc/
         'g++'
+      elsif ENV['HOMEBREW_CC'] =~ /gcc-4.2/
+        'g++-4.2'
       else
         'clang++'
       end

Library/ENV/4.3/g++-4.2

@@ -0,0 +1 @@
+cc

Library/ENV/4.3/gcc-4.2

@@ -0,0 +1 @@
+cc

Library/Homebrew/superenv.rb

@@ -77,7 +77,8 @@ class << ENV
 
   def determine_cc
     if ARGV.include? '--use-gcc'
-      "gcc"
+      # fall back to something else on systems without Apple gcc
+      MacOS.locate('gcc-4.2') ? "gcc-4.2" : raise("gcc-4.2 not found!")
     elsif ARGV.include? '--use-llvm'
       "llvm-gcc"
     elsif ARGV.include? '--use-clang'
@@ -219,6 +220,7 @@ class << ENV
   def compiler
     case ENV['HOMEBREW_CC']
       when "llvm-gcc" then :llvm
+      when "gcc-4.2" then :gcc
       when "gcc", "clang" then ENV['HOMEBREW_CC'].to_sym
     else
       raise