use forward declaration instead of disabling COM(define CINTERFACE)

[yumetodo]

Clang with Microsoft CodeGen(July 2016) produce

error : unknown type name 'IUnknown'

such error. I don't know when the bug was fixed. However, for newer version of Clang with Microsoft Codegen, use forward declaration to avoid that.

ref: #84

• [x] #84 をclose