Separating the build setup portion from the GEMS setup block

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. https://github.com/mruby/mruby/issues/6615

lib/mruby/gem.rb

@@ -53,10 +53,8 @@ module MRuby
         return if defined?(@bins)  # return if already set up
 
         MRuby::Gem.current = self
-        MRuby::Build::COMMANDS.each do |command|
-          instance_variable_set("@#{command}", @build.send(command).clone)
-        end
-        @linker.run_attrs.each(&:clear)
+        reset_commands # for backward compatibility, reset the commands from the beginning.
+        @build_settings = nil
 
         @rbfiles = Dir.glob("#{@dir}/mrblib/**/*.rb").sort
         @objs = srcs_to_objs("src")
@@ -192,6 +190,19 @@ module MRuby
         end
       end
 
+      def build_settings(&blk)
+        @build_settings = blk
+      end
+
+      def setup_build
+        if @build_settings
+          # by this point, build.cc or other commands may have been modified.
+          # therefore, reset the commands again before calling build_settings.
+          reset_commands
+          @build_settings.call(self)
+        end
+      end
+
       def define_gem_init_builder
         file "#{build_dir}/gem_init.c" => [build.mrbcfile, __FILE__] + [rbfiles].flatten do |t|
           mkdir_p build_dir
@@ -303,6 +314,13 @@ module MRuby
 
         self
       end
+
+      private def reset_commands
+        MRuby::Build::COMMANDS.each do |command|
+          instance_variable_set("@#{command}", @build.send(command).clone)
+        end
+        @linker.run_attrs.each(&:clear)
+      end
     end # Specification
 
     class Version
@@ -410,6 +428,11 @@ module MRuby
         end until gemset == gemset_prev
       end
 
+      def setup_build
+        each(&:setup_build)
+        self
+      end
+
       def setup_dependencies(build)
         gem_table = each_with_object({}) { |spec, h| h[spec.name] = spec }