Remove use of define_singleton_method.

tasks/toolchains/gcc.rake

@@ -30,10 +30,11 @@ MRuby::Toolchain.new(:gcc) do |conf|
   end
 
   [[conf.cc, 'c'], [conf.cxx, 'c++']].each do |cc, lang|
-    cc.define_singleton_method(:header_search_paths) do
+    cc.instance_variable_set :@header_search_language, lang
+    def cc.header_search_paths
       if @header_search_command != command
-        result = `echo | #{build.filename command} -x#{lang} -Wp,-v - -fsyntax-only 2>&1`
-        result = `echo | #{command} -x#{lang} -Wp,-v - -fsyntax-only 2>&1` if $?.exitstatus != 0
+        result = `echo | #{build.filename command} -x#{@header_search_language} -Wp,-v - -fsyntax-only 2>&1`
+        result = `echo | #{command} -x#{@header_search_language} -Wp,-v - -fsyntax-only 2>&1` if $?.exitstatus != 0
         return include_paths if  $?.exitstatus != 0
 
         @frameworks = []