librt links against libpthread, mostly because aio_* are implemented in threads instead of in the kernel AFAICT. This is unfortunate, because many libraries would like to use (for example) clock_gettime, but doing so pulls in pthreads. Now you have no choice but to hit the more expensive thread-safe paths, because your library can't know whether multiple threads actually exist, only whether the pthread symbols are non-zero.
The complementary problem also exists for applications that are unthreaded: if they link against librt, pthread symbols will be non-zero, and any thread-safe library beneath that app now has to behave thread-safely.
This is especially comic when reading the clock_gettime implementation, which goes out of its way to work correctly regardless of whether __pthread_clock_gettime is available.
Ideally it would be possible to provide most of these symbols from libc directly (presumably versioned so the app requires a sufficiently new libc).
The clock_* function have been moved to libc.
*** This bug has been marked as a duplicate of bug 14743 ***