Merge pull request #5325 from dearblue/c++-exc.2

`compile_as_cxx` should generate a path under` conf.build_dir`

lib/mruby/build.rb

@@ -248,7 +248,7 @@ module MRuby
           end
         end
       else
-        cxx_src = "#{build_dir}/#{src.relative_path})".ext << "-cxx.cxx"
+        cxx_src = "#{build_dir}/#{src.relative_path.to_s.remove_leading_parents}".ext << "-cxx.cxx"
         obj = cxx_src.ext(@exts.object)
       end
 

lib/mruby/core_ext.rb

@@ -18,6 +18,10 @@ class String
   def relative_path
     relative_path_from(Dir.pwd)
   end
+
+  def remove_leading_parents
+    Pathname.new(".#{Pathname.new("/#{self}").cleanpath}").cleanpath.to_s
+  end
 end
 
 def install_D(src, dst)