Change the position of -L option from MRUBY_LIBS to MRUBY_LDFLAGS
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