bug fix: the argument of '--sysroot' option should be followed '=' character.
bug fix: add new option '-mandroid' into CFLAGS and same options are applied into LDFLAGS.
refactoring
define and use print format macros for mrb_int

For portability: %ld can't be used to print a 64-bit mrb_int on
WIN(32|64) because long is 32-bit wide.
add a new feature to select the target architecture.
add a new feature to select the target Android platform.
more CFLAGS and LDFLAGS are specified automatically.
Change default gem path.
Merge pull request #974 from masuidrive/change_default_gem_path

Change default gem path.
Fix indent in string.c
Fix typo
return value from mrb_gc_arena_save() is int
restore arena after mrb_define_method()
Merge branch 'master' of github.com:mruby/mruby
Merge pull request #978 from carsonmcdonald/timetypofix

Fix typo
Merge pull request #976 from kano4/dev

Fix indent in string.c
fix inspect method
Merge pull request #981 from iij/pr-fix-inspect

fix inspect method
Rename STR_BUF_MIN_SIZE to MRB_STR_BUF_MIN_SIZE. Make it configurable.
Fix indents.