This issue was solved with changeset r38505.
Naohisa, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.
* ext/fiddle/extconf.rb, ext/fiddle/function.c
(Fiddle::Function::STDCALL): FFI_STDCALL is not a macro, but an
enumeration. [ruby-core:50398] [Bug #7483]