strip ccache from CC and CXX from rbconfig.rb

--- ruby-1.9.3-p429/tool/mkconfig.rb~	2013-06-27 19:03:54.455442002 +0300
+++ ruby-1.9.3-p429/tool/mkconfig.rb	2013-06-27 19:03:25.464126000 +0300
@@ -85,6 +85,7 @@
     when /^sitearchdir$/; sitearchdir_override = val; next
     when /^vendorarchdir$/; vendorarchdir_override = val; next
     when /^sitearch/; val = '$(arch)' if val.empty?
+    when /^(CC|CXX)$/; val.sub!(/ccache /, '')
     end
     case val
     when /^\$\(ac_\w+\)$/; next