Implement gembox-relative gemdir paths

In the case where a relative path is specified to a gembox from
build_config.rb, it was previously tricky to specify relative gem paths
from inside that gembox.

For example, consider a project in which mruby is checked out as a
submodule in the project root:

	+- project_root
	   |
	   +- mruby/
	   |  |
	   |  +- build_config.rb
	   |  |
	   |  +- ...
	   |
	   +- my_gembox/
	      |
	      +- my_gembox.gembox
	      |
	      +- my_gem/
	         |
	         +- mrbgem.rake
	         |
	         +- ...

If build_config.rb refers to my_gembox with a relative path, it's
difficult for my_gembox to then refer to my_gem. With this proposed
change, my_gembox.gembox can look like this:

	MRuby::GemBox.new do |conf|
	  conf.gem "my_gem"
	end

1 files changed, 2 insertions, 0 deletions

diff --git a/tasks/mrbgem_spec.rake b/tasks/mrbgem_spec.rake
index 94f24af26..70c44ed5e 100644
--- a/tasks/mrbgem_spec.rake
+++ b/tasks/mrbgem_spec.rake
@@ -297,6 +297,8 @@ module MRuby
 
   GemBox = Object.new
   class << GemBox
+    attr_accessor :path
+
     def new(&block); block.call(self); end
     def config=(obj); @config = obj; end
     def gem(gemdir, &block); @config.gem(gemdir, &block); end