Make it possible that `libmruby.a` is not created

Previously, `libmruby.a` was created even if only `mruby-bin-mrbc` or`
mruby-compiler` was specified for gem, but by specifying `disable_libmruby`,
the creation of `libmruby.a` can be suppressed.

### Note

The https://github.com/mruby/mruby/pull/5084#issuecomment-723521971
incompatibility seems to be difficult for users to avoid, so the original
behavior has been restored. Therefore, if we need `mrbc` other than the
`host` build, we need to explicitly specify `mruby-bin-mrbc` gem.

Due to the above changes, `build_config/boxing.rb` etc. will not work, but I
have added` mruby-bin-mrbc` to `default.gembox` to fix it. I don't think
this change is a big deal because originally `mruby-compiler` was included.

1 files changed, 3 insertions, 4 deletions

diff --git a/mrbgems/mruby-bin-mrbc/mrbgem.rake b/mrbgems/mruby-bin-mrbc/mrbgem.rake
index 48b67aedb..67200b3f0 100644
--- a/mrbgems/mruby-bin-mrbc/mrbgem.rake
+++ b/mrbgems/mruby-bin-mrbc/mrbgem.rake
@@ -2,15 +2,14 @@ MRuby::Gem::Specification.new 'mruby-bin-mrbc' do |spec|
   spec.license = 'MIT'
   spec.author  = 'mruby developers'
   spec.summary = 'mruby compiler executable'
-
   spec.add_dependency 'mruby-compiler', :core => 'mruby-compiler'
 
   exec = exefile("#{build.build_dir}/bin/mrbc")
-  mrbc_objs = Dir.glob("#{spec.dir}/tools/mrbc/*.c").map { |f| objfile(f.pathmap("#{spec.build_dir}/tools/mrbc/%n")) }.flatten
+  mrbc_objs = Dir.glob("#{spec.dir}/tools/mrbc/*.c").map { |f| objfile(f.pathmap("#{spec.build_dir}/tools/mrbc/%n")) }
 
-  file exec => mrbc_objs + [build.libmruby_core_static] do |t|
+  file exec => mrbc_objs << build.libmruby_core_static do |t|
     build.linker.run t.name, t.prerequisites
   end
 
-  build.bins << 'mrbc' unless build.bins.find { |v| v == 'mrbc' }
+  build.bins << 'mrbc'
 end