Add missing build_config.rb

third-party/build_config.rb

+MRuby::Build.new do |conf|
+  # TODO use same compilers configured in configure script
+  toolchain :clang
+
+  # C++ project needs this.  Without this, mruby exception does not
+  # properly destory C++ object allocated on stack.
+  conf.enable_cxx_abi
+
+  conf.build_dir = ENV['BUILD_DIR']
+
+  # include the default GEMs
+  conf.gembox 'default'
+  conf.gem :core => 'mruby-eval'
+end