Update build process for both host and cross compile.

lib/mruby/build.rb

@@ -81,7 +81,8 @@ module MRuby
         @mrbc = Command::Mrbc.new(self)
 
         @bins = []
-        @gems, @libmruby_objs = MRuby::Gem::List.new, []
+        @gems = MRuby::Gem::List.new
+        @libmruby_objs = []
         @build_mrbtest_lib_only = false
         @cxx_exception_enabled = false
         @cxx_exception_disabled = false
@@ -95,11 +96,10 @@ module MRuby
         MRuby.targets[@name] = self
       end
 
-      MRuby::Build.current = MRuby.targets[@name]
-      MRuby.targets[@name].instance_eval(&block)
-
-      build_mrbc_exec if name == 'host'
-      build_mrbtest if test_enabled?
+      current = MRuby.targets[@name]
+      MRuby::Build.current = current
+      current.instance_eval(&block)
+      current.build_mrbtest if current.test_enabled?
     end
 
     def debug_enabled?
@@ -247,8 +247,9 @@ EOS
     def mrbcfile
       return @mrbcfile if @mrbcfile
 
-      unless gems.detect { |v| v.name == 'mruby-bin-mrbc' }
-        gem :core => "mruby-bin-mrbc"
+      unless gems.detect {|v| v.name == 'mruby-bin-mrbc' }
+        build_mrbc_exec
+        gems.detect {|v| v.name == 'mruby-bin-mrbc' }.setup
       end
       @mrbcfile = self.exefile("#{self.build_dir}/bin/mrbc")
     end

lib/mruby/build/load_gems.rb

@@ -28,18 +28,19 @@ module MRuby
       Gem.current = nil
       load gemrake
       return nil unless Gem.current
+      current = Gem.current
 
-      Gem.current.dir = gemdir
-      Gem.current.build = self.is_a?(MRuby::Build) ? self : MRuby::Build.current
-      Gem.current.build_config_initializer = block
-      gems << Gem.current
+      current.dir = gemdir
+      current.build = self.is_a?(MRuby::Build) ? self : MRuby::Build.current
+      current.build_config_initializer = block
+      gems << current
 
       cxx_srcs = ['src', 'test', 'tools'].map do |subdir|
-        Dir.glob("#{Gem.current.dir}/#{subdir}/*.{cpp,cxx,cc}")
+        Dir.glob("#{current.dir}/#{subdir}/*.{cpp,cxx,cc}")
       end.flatten
       enable_cxx_exception unless cxx_srcs.empty?
 
-      Gem.current
+      current
     end
 
     def load_special_path_gem(params)

lib/mruby/gem.rb

@@ -47,6 +47,8 @@ module MRuby
         @version = "0.0.0"
         @mrblib_dir = "mrblib"
         @objs_dir = "src"
+        @dependencies = []
+        @conflicts = []
         MRuby::Gem.current = self
       end
 
@@ -75,7 +77,6 @@ module MRuby
         @bins = []
 
         @requirements = []
-        @dependencies, @conflicts = [], []
         @export_include_paths = []
         @export_include_paths << "#{dir}/include" if File.directory? "#{dir}/include"