fix argument spec setting of Kernel.exit method.
this method requires 1 optional argument.
Merge pull request #1806 from tmash06/fix_exit_method_aspec_setting

fix argument spec setting of Kernel.exit method.
fix mruby command returning EXIT_SUCCESS when program file can't be
opened.
remove unused variable

GLOBAL_RAND_SEED_KEY is no longer used.
Merge pull request #1807 from tmash06/fix_exit_status_code

fix mruby command returning EXIT_SUCCESS when program file can't be opened.
Change the position of -L option from MRUBY_LIBS to MRUBY_LDFLAGS
Merge pull request #1808 from cubicdaiya/issues/unused_variable

remove unused variable
Merge pull request #1809 from matsumoto-r/change_mruby_libs_flag

Change the position of -L option from MRUBY_LIBS to MRUBY_LDFLAGS
use mrb_int instead of size_t since string length is represented by mrb_int; close #1810
Merge branch 'master' of github.com:mruby/mruby
small refactor
Merge pull request #1811 from takkaw/refactor_mirb

small refactor in mirb
mruby-time: remove trailing whitespace
mruby-time: rearrange header order
mruby-time: fix two tiny typos in comments
mruby-time: mrb_time_type should be const
mruby-time: store strings directly instead of pointers to them
mruby-time: timezone_names should be const
mruby-time: store timezone name directly
mruby-time: fix indentation in mrb_time_asctime