Warn user if they set CFLAGS, etc. with superenv

Since we remove some CFLAGS under their noses, this would otherwise be quite confusing.

Notably, this will now trigger in numerous formula. Sucks.

Library/ENV/4.3/cc

@@ -135,6 +135,10 @@ end
 ####################################################################### sanity
 abort "The build-tool has reset ENV. --lame-env required." unless ENV['HOMEBREW_BREW_FILE']
 
+%w{CPPFLAGS LDFLAGS CXXFLAGS CFLAGS}.each do |flag|
+  puts "#{flag} set but superenv running! Flags may be removed!" if ENV[flag]
+end
+
 ######################################################################### main
 cmd = Cmd.new($0, ARGV)
 exec "xcrun", cmd.tool, *cmd.args

Library/Homebrew/build.rb

@@ -57,7 +57,9 @@ def install f
   # Python etc. build but then pip can't build stuff.
   # Scons resets ENV and then can't find superenv's build-tools.
   stdenvs = %w{fontforge python python3 ruby ruby-enterprise-edition jruby}
-  ARGV.unshift '--env=std' if stdenvs.include?(f.name) or f.recursive_deps.detect{|d| d.name == 'scons' }
+  ARGV.unshift '--env=std' if (stdenvs.include?(f.name) or
+    f.recursive_deps.detect{|d| d.name == 'scons' }) and
+    not ARGV.include? '--env=super'
 
   keg_only_deps = f.recursive_deps.uniq.select{|dep| dep.keg_only? }
 
@@ -82,6 +84,14 @@ def install f
   if superenv?
     ENV.deps = keg_only_deps.map(&:to_s)
     ENV.setup_build_environment
+    class << ENV
+      def []=(key, value)
+        case key when 'CFLAGS', 'CPPFLAGS', 'LDFAGS'
+          opoo "#{key} set with --env=super! Flags may not take effect!"
+        end
+        store(key, value)
+      end
+    end
   end
 
   f.recursive_requirements.each { |req| req.modify_build_environment }