It's not a change in glibc.  __pthread_cond_timedwait is the internal function name while pthread_cond_timedwait is the exported alias.  You're seeing __pthread_cond_timedwait here because either your glibc installation has debug symbols or you have debug info packages installed, which allows gdb to resolve the function name to the internal name.  Without glibc debug info you should see just pthread_cond_timedwait@@... or just pthread_cond_timedwait.

In any case, I guess you'd be better off just using .find("pthread_cond_timedwait") instead of startswith() since I've also seen this on ppc64, which might break your test again:

Thread 2 (Thread 0x3fffb7d1f200 (LWP 5746)):
#0  0x00003fffb7f21ec8 in .pthread_cond_timedwait () from /lib64/libpthread.so.0

I'm not entirely sure what the preceding dot means, but it seems to indicate a function call outside the binary in ppc64.