The issue resolved by the preceding patch was solely the C++ exception task within the mruby core.
This patch aims to resolve a similar sequencing issue that also exists in GEMS.
In practice, `mruby-compiler` is sometimes loaded via dependencies rather than being explicitly specified in the build configuration file.
In such cases, when `mruby-compiler/mrbgem.rake` is loaded, it is not yet determined whether C++ exceptions will be used. Consequently, even if it later becomes clear that `core/codegen-cxx.cxx` and `core/y.tab-cxx.cxx` are required, the system could not handle this.
To resolve this issue, we introduce the `MRuby::Gem::Specification#build_settings` method as a mechanism for lazily evaluating build setup.
However, for backward compatibility, the commands are cloned twice in `gem.setup` and `gem.setup_build`.
This is because many existing GEMS configure commands directly within the setup block.
ref. mruby#6615