Log Message:
Update to 6.39. From the changelog:

- work around more incompatible changes in 5.20.
- check that perl slots actually have enough space to hold
  interpreter variables.
- untested port to perl 5.19 (64 bit tmps indices) (reported
  by Andreas König).
- croak when cancel is called without a thread context.
- *sigh*, removed leftover debugging code from debugging a
  perl bug, of all things.
- semaphores would not clear the destroy hook when interrupted
  by ->throw, causing segfaults or worse.
- ->throw on a thread waiting for a semaphore did not acquire
  the semaphore, but also didn't wake up other waiters,
  possibly causing a deadlock.
- "FATAL: $Coro::IDLE blocked itself" will now use Carp::confess
  to report a full stacktrace, which should help find out
  where the actual call is.
- "atomically" destroy data in slf_destroy, because it is
  the right thing to do, just in case.
- disable ecb.h libm dependency, to compile on ancient systems
  or under adverse conditions.
- do not crash when freeing padlists with holes (in 5.18).
- tentative SVt_BIND 5.19 port/fix.
- use a new algorithm to derive padlists for perl 5.18. The old
  one could lead to 0-pointer accesses inside perl (reported
  by Darin McBride).

Not a leaf, but this update should fix a handful of depending
packages, and at least one of those passes its tests against this
version.