This is so that the build directory can be regarded as a temporary installation directory to work in.

Directories set in `MRuby::Gem::Specification#export_include_paths` are used as they are if they are subdirectories placed under `gem.dir`.
Files are placed under `<build-dir>/include/mruby/gems/<gem-name>/<gem-include_paths>` to separate each GEM.

When GEM is compiled by building `libmruby.a`, the same `include_paths` will be set as before, so it is expected that no unintended references will be made.