Update `Rakefile`.

So that you can omit `host` target. Now `host-debug` works.

Rakefile

@@ -52,15 +52,19 @@ task :default => :all
 
 bin_path = ENV['INSTALL_DIR'] || "#{MRUBY_ROOT}/bin"
 
-depfiles = MRuby.targets['host'].bins.map do |bin|
-  install_path = MRuby.targets['host'].exefile("#{bin_path}/#{bin}")
-  source_path = MRuby.targets['host'].exefile("#{MRuby.targets['host'].build_dir}/bin/#{bin}")
-
-  file install_path => source_path do |t|
-    install_D t.prerequisites.first, t.name
+if MRuby.targets['host']
+  target = MRuby.targets['host']
+  depfiles = target.bins.map do |bin|
+    install_path = target.exefile("#{bin_path}/#{bin}")
+    source_path = target.exefile("#{target.build_dir}/bin/#{bin}")
+
+    file install_path => source_path do |t|
+      install_D t.prerequisites.first, t.name
+    end
+    install_path
   end
-
-  install_path
+else
+  depfiles = []
 end
 
 MRuby.each_target do |target|

lib/mruby/build.rb

@@ -247,9 +247,10 @@ EOS
     def mrbcfile
       return @mrbcfile if @mrbcfile
 
-      mrbc_build = MRuby.targets['host']
-      gems.each { |v| mrbc_build = self if v.name == 'mruby-bin-mrbc' }
-      @mrbcfile = mrbc_build.exefile("#{mrbc_build.build_dir}/bin/mrbc")
+      unless gems.detect { |v| v.name == 'mruby-bin-mrbc' }
+        gem :core => "mruby-bin-mrbc"
+      end
+      @mrbcfile = self.exefile("#{self.build_dir}/bin/mrbc")
     end
 
     def compilers
@@ -387,7 +388,8 @@ EOS
     end
 
     def mrbcfile
-      MRuby.targets['host'].exefile("#{MRuby.targets['host'].build_dir}/bin/mrbc")
+      host = MRuby.targets['host']
+      host.exefile("#{host.build_dir}/bin/mrbc")
     end
 
     def run_test