Correctly check for C++17 and higher when declaring timespec_get()

Summary:
In rS338751, the check to declare timespec_get() for C++17 and higher
was incorrectly done against a cplusplus define, while it should have
been __cplusplus.

Fix this by using __cplusplus, and also bump __FreeBSD_version so it
becomes possible to correctly check for timespec_get() in upstream
libc++ headers.

Reviewed by: brooks, emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D22735