Scupper sourceable output if brew install is piped

`brew install > file` should not give "export CC=/foo" output on Build Failures.

Also moved the "Using clang" output to brew when BuildError is thrown as that is where it is interesting, and not if you just do `brew --env`.

Library/Homebrew/cmd/--env.rb

@@ -6,38 +6,35 @@ module Homebrew extend self
     ENV.extend(HomebrewEnvExtension)
     ENV.setup_build_environment
     ENV.universal_binary if ARGV.build_universal?
-    dump_build_env ENV
+    if $stdout.tty?
+      dump_build_env ENV
+    else
+      build_env_keys(ENV).each do |key|
+        puts "export #{key}=\"#{ENV[key]}\""
+      end
+    end
+  end
+
+  def build_env_keys env
+    %w[ CC CXX LD CFLAGS CXXFLAGS CPPFLAGS LDFLAGS MACOSX_DEPLOYMENT_TARGET
+      MAKEFLAGS PKG_CONFIG_PATH HOMEBREW_BUILD_FROM_SOURCE HOMEBREW_DEBUG
+      HOMEBREW_MAKE_JOBS HOMEBREW_VERBOSE HOMEBREW_USE_CLANG HOMEBREW_USE_GCC
+      HOMEBREW_USE_LLVM HOMEBREW_SVN ].select{ |key| env[key] }
   end
 
   def dump_build_env env
-    puts %["--use-clang" was specified] if ARGV.include? '--use-clang'
-    puts %["--use-llvm" was specified] if ARGV.include? '--use-llvm'
-    puts %["--use-gcc" was specified] if ARGV.include? '--use-gcc'
-
-    %w[ CC CXX LD ].each do |k|
-      value = env[k]
-      if value and $stdout.tty?
-        results = value
+    build_env_keys(env).each do |key|
+      value = env[key]
+      print "#{key}: #{value}"
+      case key when 'CC', 'CXX', 'LD'
         if value =~ %r{/usr/bin/xcrun (.*)}
           path = `/usr/bin/xcrun -find #{$1}`
-          results += " => #{path}"
-        elsif File.exists? value and File.symlink? value
-          results += " => #{Pathname.new(value).realpath}"
+          print " => #{path}"
+        elsif File.symlink? value
+          print " => #{Pathname.new(value).realpath}"
         end
-        env[k] = results
       end
-    end
-
-    %w[ CC CXX LD CFLAGS CXXFLAGS CPPFLAGS LDFLAGS MACOSX_DEPLOYMENT_TARGET MAKEFLAGS PKG_CONFIG_PATH
-        HOMEBREW_BUILD_FROM_SOURCE HOMEBREW_DEBUG HOMEBREW_MAKE_JOBS HOMEBREW_VERBOSE
-        HOMEBREW_USE_CLANG HOMEBREW_USE_GCC HOMEBREW_USE_LLVM HOMEBREW_SVN ].each do |k|
-      value = env[k]
-      out = if $stdout.tty?
-        "#{k}: #{value}"
-      else
-        "export #{k}=\"#{value}\""
-      end
-      puts out if value
+      puts
     end
   end
 end

bin/brew

@@ -118,6 +118,9 @@ rescue BuildError => e
   ohai "Environment"
   puts Homebrew.config_s
   ohai "Build Flags"
+  puts %["--use-clang" was specified] if ARGV.include? '--use-clang'
+  puts %["--use-llvm" was specified] if ARGV.include? '--use-llvm'
+  puts %["--use-gcc" was specified] if ARGV.include? '--use-gcc'
   Homebrew.dump_build_env e.env
   puts
   onoe e