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, 1 insertions, 1 deletions

diff --git a/Rakefile b/Rakefile
index c2c76ba1b..e54983f41 100644
--- a/Rakefile
+++ b/Rakefile
@@ -230,7 +230,7 @@ desc "preallocated symbols"
 task :gensym => presym_file
 
 depfiles += MRuby.targets.map { |n, t|
-  t.libraries
+  t.libmruby_enabled? ? t.libraries : t.libmruby_core_static
 }.flatten
 
 depfiles += MRuby.targets.reject { |n, t| n == 'host' }.map { |n, t|