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