gc.c: suppress a warning

* gc.c (get_envparam_size): suppress a warning by char-subscripts.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45613 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
ruby.c: drop older cygwin

* ruby.c (push_include_cygwin): drop older cygwin support.

* ruby.c (ruby_init_loadpath_safe): ditto, and always use String as
  libpath buffer on Windows.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45614 b2dd03c8-39d4-4d8f-98ff-823fe69b080e