Refactor `src/mruby_core.rake`

src/mruby_core.rake

-MRuby.each_target do
-  current_dir = File.dirname(__FILE__).relative_path_from(Dir.pwd)
-  relative_from_root = File.dirname(__FILE__).relative_path_from(MRUBY_ROOT)
-  current_build_dir = "#{build_dir}/#{relative_from_root}"
-
-  objs = Dir.glob("#{current_dir}/*.c").map { |f|
-    next nil if cxx_exception_enabled? and f =~ /(error|vm|gc)\.c$/
-    objfile(f.pathmap("#{current_build_dir}/%n"))
-  }.compact
+as_cxx_srcs = %w[vm error gc].map{|name| "#{MRUBY_ROOT}/src/#{name}.c"}
 
-  if cxx_exception_enabled?
-    objs += %w(vm error gc).map { |v| compile_as_cxx "#{current_dir}/#{v}.c", "#{current_build_dir}/#{v}.cxx" }
+MRuby.each_target do
+  objs = Dir.glob("#{MRUBY_ROOT}/src/*.c").map do |src|
+    dst = src.pathmap("#{build_dir}/src/%n")
+    if cxx_exception_enabled? && as_cxx_srcs.include?(src)
+      compile_as_cxx(src, "#{dst}.cxx")
+    else
+      objfile(dst)
+    end
   end
   self.libmruby_objs << objs