vm_method.c: configurable global method cache size

* vm_method.c (Init_Method): make global method cache size
  configurable by environment variable
  "RUBY_GLOBAL_METHOD_CACHE_SIZE"  [Fix GH-719]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47621 b2dd03c8-39d4-4d8f-98ff-823fe69b080e