With xcodebuild don't set CC, CXX, etc.

The compiler setting in the xcodeproj is overridden by the CC setting in the environment. This is a bit insane IMO since the xcodeproj itself has detailed decisions about what tools to use.

Fixes Homebrew/homebrew#6406.

Library/Homebrew/extend/ENV.rb

@@ -351,4 +351,13 @@ Please take one of the following actions:
       Hardware.processor_count
     end
   end
+
+  def remove_cc_etc
+    keys = %w{CC CXX LD CPP LDFLAGS CFLAGS CPPFLAGS}
+    removed = Hash[*keys.map{ |key| [key, ENV[key]] }.flatten]
+    keys.each do |key|
+      ENV[key] = nil
+    end
+    removed
+  end
 end

Library/Homebrew/formula.rb

@@ -488,6 +488,11 @@ protected
     pretty_args.delete "--disable-dependency-tracking" if cmd == "./configure" and not ARGV.verbose?
     ohai "#{cmd} #{pretty_args*' '}".strip
 
+    removed_ENV_variables = case if args.empty? then cmd.split(' ').first else cmd end
+    when "xcodebuild"
+      ENV.remove_cc_etc
+    end
+
     if ARGV.verbose?
       safe_system cmd, *args
     else
@@ -508,6 +513,11 @@ protected
         raise
       end
     end
+
+    removed_ENV_variables.each do |key, value|
+      ENV[key] = value # ENV.kind_of? Hash  # => false
+    end if removed_ENV_variables
+
   rescue
     raise BuildError.new(self, cmd, args, $?)
   end