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.

Build targets other than "host" will be installed under `<PREFIX>/mruby/<build-name>`.
This can be changed with `build.install_prefix = dir`.

If the environment variable `DESTDIR` is set, the writing destination is `<DESTDIR>/<PREFIX>/...`.
It is assumed to be used for package work.

This is a complement to mruby#5928.
The previous PR had the following problem:
  - The `<INSTALL_DIR>/bin/*` file could not be replaced if the destination of the symbolic link was lost.
  - The wrong link destination was written if `MRuby::Build.install_dir` was a relative path.