Add default `host` target for cross compiling.

You don't have to define explicit `host` build target any more.

lib/mruby/build.rb

@@ -370,6 +370,17 @@ EOS
     attr_accessor :host_target, :build_target
 
     def initialize(name, build_dir=nil, &block)
+      unless MRuby.targets['host']
+        # add minimal 'host'
+        MRuby::Build.new('host') do |conf|
+          if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
+            toolchain :visualcpp
+          else
+            toolchain :gcc
+          end
+          conf.gem :core => 'mruby-bin-mrbc'
+        end
+      end
       @endian = nil
       @test_runner = Command::CrossTestRunner.new(self)
       super

target/ArduinoDue.rb

-MRuby::Build.new do |conf|
-
-  # Gets set by the VS command prompts.
-  if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
-    toolchain :visualcpp
-  else
-    toolchain :gcc
-  end
-
-  enable_debug
-
-  # include the default GEMs
-  conf.gembox 'default'
-
-end
-
 # Cross Compiling configuration for Arduino Due
 # http://arduino.cc/en/Main/ArduinoBoardDue
 #

target/IntelGalileo.rb

-MRuby::Build.new do |conf|
-
-  # Gets set by the VS command prompts.
-  if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
-    toolchain :visualcpp
-  else
-    toolchain :gcc
-  end
-
-  enable_debug
-
-  # include the default GEMs
-  conf.gembox 'default'
-
-end
-
-
 # Cross Compiling configuration for Intel Galileo on Arduino environment
 # http://arduino.cc/en/ArduinoCertified/IntelGalileo
 #

target/RX630.rb

-MRuby::Build.new do |conf|
-
-  # Gets set by the VS command prompts.
-  if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
-    toolchain :visualcpp
-  else
-    toolchain :gcc
-  end
-
-  enable_debug
-
-  # include the default GEMs
-  conf.gembox 'default'
-
-end
-
 # Cross Compiling configuration for RX630
 # http://gadget.renesas.com/
 #

target/android_arm64-v8a.rb

-MRuby::Build.new do |conf|
-
-  # Gets set by the VS command prompts.
-  if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
-    toolchain :visualcpp
-  else
-    toolchain :gcc
-  end
-
-  enable_debug
-
-  # include the default GEMs
-  conf.gembox 'default'
-end
-
 # Requires Android NDK r13 or later.
 MRuby::CrossBuild.new('android-arm64-v8a') do |conf|
   params = {

target/android_armeabi.rb

-MRuby::Build.new do |conf|
-
-  # Gets set by the VS command prompts.
-  if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
-    toolchain :visualcpp
-  else
-    toolchain :gcc
-  end
-
-  enable_debug
-
-  # include the default GEMs
-  conf.gembox 'default'
-end
-
 # Requires Android NDK r13 or later.
 MRuby::CrossBuild.new('android-armeabi') do |conf|
   params = {

target/android_armeabi_v7a_neon_hard.rb

-MRuby::Build.new do |conf|
-
-  # Gets set by the VS command prompts.
-  if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
-    toolchain :visualcpp
-  else
-    toolchain :gcc
-  end
-
-  enable_debug
-
-  # include the default GEMs
-  conf.gembox 'default'
-end
-
 # Requires Android NDK r13 or later.
 MRuby::CrossBuild.new('android-armeabi-v7a-neon-hard') do |conf|
   params = {

target/chipKITMax32.rb

-MRuby::Build.new do |conf|
-
-  # Gets set by the VS command prompts.
-  if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
-    toolchain :visualcpp
-  else
-    toolchain :gcc
-  end
-
-  enable_debug
-
-  # include the default GEMs
-  conf.gembox 'default'
-
-end
-
 # Cross Compiling configuration for Digilent chipKIT Max32
 # http://www.digilentinc.com/Products/Detail.cfm?Prod=CHIPKIT-MAX32
 #

target/cross-32bit.rb

 # Define cross build settings
-MRuby::CrossBuild.new('32bit') do |conf|
+MRuby::CrossBuild.new('cross-32bit') do |conf|
   toolchain :gcc
 
   conf.cc.flags << "-m32"
   conf.linker.flags << "-m32"
 
-  conf.build_mrbtest_lib_only
+  # conf.build_mrbtest_lib_only
 
-  conf.gem 'examples/mrbgems/c_and_ruby_extension_example'
+  conf.gem :core => "mruby-bin-mruby"
+  conf.gem "#{MRUBY_ROOT}/examples/mrbgems/c_and_ruby_extension_example"
 
   conf.test_runner.command = 'env'
 end

target/dreamcast_shelf.rb

-MRuby::Build.new do |conf|
-  # Gets set by the VS command prompts
-  if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
-    toolchain :visualcpp
-  else
-    toolchain :gcc
-  end
-
-  enable_debug
-
-  # Include the default GEMs
-  conf.gembox 'default'
-end
-
 # Cross Compiling configuration for the Sega Dreamcast
 # This configuration requires KallistiOS (KOS)
 # https://dreamcast.wiki