Search gem from MRUBY_ROOT if build config is in build_config.

doc/guides/mrbgems.md

@@ -12,12 +12,18 @@ a GEM, add the following line to your build configuration file, for example:
 conf.gem '/path/to/your/gem/dir'
 ```
 
-You can also use a relative path which would be relative from the mruby root:
+You can also use a relative path to specify a gem.
 
 ```ruby
 conf.gem 'examples/mrbgems/ruby_extension_example'
 ```
 
+In that case, 
+
+* if your build configuration file is in the `build_config` directory, it's
+  relative from `MRUBY_ROOT`.
+* otherwise, it is relative from the directory where your build configuration is.
+
 A remote GIT repository location for a GEM is also supported:
 ```ruby
 conf.gem :git => 'https://github.com/masuidrive/mrbgems-example.git', :branch => 'master'

lib/mruby/build/load_gems.rb

@@ -15,10 +15,14 @@ module MRuby
     def gem(gemdir, &block)
       if gemdir.is_a?(Hash)
         gemdir = load_special_path_gem(gemdir)
-      elsif GemBox.path && gemdir.is_a?(String)
+      elsif GemBox.path
         gemdir = File.expand_path(gemdir, File.dirname(GemBox.path))
       else
-        gemdir = File.expand_path(gemdir, MRUBY_ROOT)
+        caller_dir = File.expand_path(File.dirname(caller(1,1)[0][/^(.*?):\d/,1]))
+        if caller_dir == "#{MRUBY_ROOT}/build_config"
+          caller_dir = MRUBY_ROOT
+        end
+        gemdir = File.expand_path(gemdir, caller_dir)
       end
 
       gemrake = File.join(gemdir, "mrbgem.rake")