Build process updated:

You have to specify `TARGET` to specify a configuration, e.g.

```
rake TARGET=host-debug all test
```

When you port `mruby` to a new configuration:
1. copy an existing configuration under `target` directory
2. modify the new configuration file
3. build using the new configuration
4. send PR if you please

1 files changed, 24 insertions, 0 deletions

diff --git a/target/host-debug.rb b/target/host-debug.rb
new file mode 100644
index 000000000..4e89d162c
--- /dev/null
+++ b/target/host-debug.rb
@@ -0,0 +1,24 @@
+MRuby::Build.new('host-debug') do |conf|
+  # load specific toolchain settings
+
+  # 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 'full-core'
+
+  # C compiler settings
+  conf.cc.defines = %w(MRB_ENABLE_DEBUG_HOOK)
+
+  # Generate mruby debugger command (require mruby-eval)
+  conf.gem :core => "mruby-bin-debugger"
+
+  # bintest
+  # conf.enable_bintest
+end