Unable to bundle gem

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I have a very minimalist build_config.rb to test that:
```ruby
MRuby::Build.new do |conf|
if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
toolchain :visualcpp
else
toolchain :gcc
end

enable_debug

conf.gembox 'default'

conf.gem 'path/to/mruby-gem_a'

conf.gem :github => 'mattn/mruby-require'

conf.gem 'path/to/mruby-gem_b'
```

Here `mruby-gem_a` and `mruby-gem_b` are just dummy names.
I tried using minirake and rake, the same problem arise:
```
Build summary:

================================================
Config Name: host
Output Directory: build/x86/host
Binaries: mrbc
Included Gems:
mruby-dir
mruby-io - IO and File class
mruby-random - Random class
mruby-sprintf - standard Kernel#sprintf method
mruby-time - standard Time class
mruby-tempfile
mruby-pack - Array#pack and String#unpack method
mruby-print - standard print/puts/p
mruby-math - standard Math module
mruby-struct - standard Struct class
mruby-compar-ext - Enumerable module extension
mruby-enum-ext - Enumerable module extension
mruby-string-ext - String class extension
mruby-numeric-ext - Numeric class extension
mruby-array-ext - Array class extension
mruby-hash-ext - Hash class extension
mruby-range-ext - Range class extension
mruby-proc-ext - Proc class extension
mruby-symbol-ext - Symbol class extension
mruby-object-ext - Object class extension
mruby-objectspace - ObjectSpace class
mruby-fiber - Fiber class
mruby-enumerator - Enumerator class
mruby-enum-lazy - Enumerator::Lazy class
mruby-toplevel-ext - toplevel object (main) methods extension
mruby-compiler - mruby compiler library
mruby-bin-mirb - mirb command
- Binaries: mirb
mruby-error - extensional error handling
mruby-bin-mruby - mruby command
- Binaries: mruby
mruby-bin-strip - irep dump debug section remover command
- Binaries: mruby-strip
mruby-kernel-ext - Kernel module extension
mruby-class-ext - class/module extension
mruby-gem_a
mruby-bin-mrbc - mruby compiler executable
mruby-require
================================================

** Invoke C:/Workspace/MRuby/build/x86/host/lib/mruby-gem_b.so (first_time)
rake aborted!
Don't know how to build task 'C:/Workspace/MRuby/build/x86/host/mrbgems/mruby-gem_b/src/mruby-gem_b.obj (see --tasks)
```

@pbosetti you made the change about that feature, right ? What am I missing here ?

Start with the minimalist build_config.rb and reproduce the failure using rake or minirake. Trace how the build handles mruby-gem_b/src/mruby-gem_b.obj after invoking build/x86/host/lib/mruby-gem_b.so; done means the gem bundle completes without the missing-task error.

Written by the indexing model from the issue text.