Fixed pure ruby mrbgems can't compile and fixed build script wrong variable name
Merge remote-tracking branch 'mruby/master'
Remove DEBUG macro definitions.
gc.c: Enable DEBUG(x) if GC_DEBUG is defined.
vm.c: Enable DEBUG(x) if VM_DEBUG is defined.
Add check for using cdump with DISABLE_STDIO option.
add test cases for GC API
Compact flags for GC
Skip sweeping old slots which don't contain any young object.
Merge pull request #701 from monaka/pr-remove-debug-macros

Remove debug macros from mruby.h
Merge pull request #703 from authorNari/skip_sweeping_old

Skip sweeping old slots which don't contain any young object in the minor GC
Merge pull request #704 from authorNari/gc_test

Add test cases for GC API
Merge pull request #705 from authorNari/compact_flags

Compact flags for GC
Added gem :github option
Merge pull request #706 from masuidrive/add_gem_github_option

Added gem :github option
don't use static class instances
Merge branch 'master' of https://github.com/mruby/mruby
Export mrb_ary_clear in array.h
Merge remote-tracking branch 'mruby/master' into array-clear
Merge pull request #707 from bovi/array-clear

Export Array#clear in array.h
Merge pull request #702 from monaka/pr-check-cdump-with-disable-stdio

Add check for using cdump with DISABLE_STDIO option.