enable THRIFT_HAS_OPTIONAL in recent MSVC without /Zc:__cplusplus
enable THRIFT_HAS_OPTIONAL in recent MSVC without /Zc:__cplusplus

Summary:
When compiling Thrift in MSVC in C++17 mode, enable
THRIFT_HAS_OPTIONAL even if /Zc:__cplusplus isn't enabled.

Reviewed By: vitaut

Differential Revision: D25046832

fbshipit-source-id: 4fe85c412f9c4faa0a5d9d2a990ccf0fed5b340d

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift