Fix LIBCPP cmake check
Fix LIBCPP cmake check

Summary:
The test was missing a `main` meaning that it would fail even if libc++ was in use, making all the rest fail on macos for example.
Closes https://github.com/facebook/folly/pull/831

Reviewed By: Orvid

Differential Revision: D7787115

Pulled By: yfeldblum

fbshipit-source-id: 4a10323298be3f26f783fa1257cbde2483fb75b9

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly