don't always generate gem functions

If the src and mrblib directories of a mrbgem don't exist or don't
include "usable" files (for tools like mirb), then functions for gem
initialization and finalization don't have to be generated.
timer on windows bug fixed.
Merge pull request #2522 from dycoon/debug_timer

timer on windows bug fixed.
Merge pull request #2503 from cremno/dont-always-generate-gem-functions

don't always generate gem functions
changed mrb_str_size and mrb_str_bytesize comments.
Fix String#succ. "-".succ should be "."
Fix String#succ. "-a-".succ should be "-b-"
"-a-a-".succ should be "-a-b-"
Merge pull request #2523 from kkkkkt/changed_string_method_comment

changed mrb_str_size and mrb_str_bytesize comments.
Merge pull request #2524 from mattn/fix-succ

Fix String#succ. "-".succ should be "."
fix build when neither src nor mrblib directory exists.
Merge pull request #2526 from iij/pr-test-only-gem

fix build when neither src nor mrblib directory exists.
mrb_str_bytesize is a carbon copy of mrb_str_size.  remove it.

mruby core does not support any character encodings but ASCII.
Merge pull request #2527 from iij/pr-remove-mrb_str_bytesize

mrb_str_bytesize is a carbon copy of mrb_str_size.  remove it.
changed some string method's mrb-aspec
Merge pull request #2529 from kkkkkt/change-string-method-mrb-aspec

changed some string method's mrb-aspec