Log Message:
Overhaul buildlink3 processing of Ruby.

* Don't buildlink in ruby/rubyversion.mk any more but define
  RUBY_USE_PTHREAD (use of pthread).
* In ruby/buildlink3.mk, buildlink via mk/pthread.buildlink3.mk as to
  RUBY_USE_PTHREAD.
* Also the same logic in ruby/Makefile.common.
* Buildlink of bdb, libiconv, zlib, openssl in each ruby*-base/Makefile.