* common.mk: generate ext_prelude.c by prelude.rb and gem_prelude.rb.
ruby (not miniruby) is linked with ext_prelude.o instead of prelude.o.
* inits.c (rb_call_inits): don't call Init_prelude.
* ruby.c: support --disable-gems option.
(ruby_init_gems): new function to define Gem::Enable and
invoke Init_prelude.
(process_options): call ruby_init_gems just after
ruby_init_loadpath.
* tool/compile_prelude.rb: support multiple files.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@13865 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
