add enable_debug method to MRuby::Build and build debug version in travis-ci

tasks/mruby_build.rake

@@ -87,6 +87,11 @@ module MRuby
       MRuby.targets[@name].instance_eval(&block)
     end
 
+    def enable_debug
+      compilers.each { |c| c.defines += %w(MRB_DEBUG) }
+      @mrbc.compile_options += ' -g'
+    end
+
     def toolchain(name)
       tc = Toolchain.toolchains[name.to_s]
       fail "Unknown #{name} toolchain" unless tc

travis_config.rb

+MRuby::Build.new('debug') do |conf|
+  toolchain :gcc
+  enable_debug
+
+  # include all core GEMs
+  conf.gembox 'full-core'
+  conf.cc.defines = %w(MRB_GC_FIXED_ARENA)
+end
+
 MRuby::Build.new do |conf|
   toolchain :gcc
 
   # include all core GEMs
   conf.gembox 'full-core'
-  conf.cc.defines = %w(MRB_DEBUG MRB_GC_FIXED_ARENA)
+  conf.cc.defines = %w(MRB_GC_FIXED_ARENA)
 end