Log Message:
Update php-apcu to 5.1.16. From Michael Moll via Github pull request.

Closes NetBSD/pkgsrc#42.

5.1.16
- Fix build on OSX.

5.1.15
- Restore apc.serializer=php as the default, as the "default" serializer
  still/again has issues.
- Fix possible issues in persistence of arrays with the "default" \ 
serializer.
- Attempt to reduce shared memory fragementation.

5.1.14
- Fixed GH #347: Disable slam defense by default.
- Fix potential issue with destruction of locks. This does not affect Linux,
  but might affect Windows and BSD.
- Use mutex instead of rwlock for shared memory allocator (if pthreads mutex
  available).
- Require only read-lock for apcu_cas(), by using atomic compare-and-swap.

5.1.13
- Reimplement persistence logic using precise allocation rather than memory
  pools. This reduces memory usage of cache entries, especially for small
  values, and improves performance of persisting and unpersisting values.
- Fixed GH #335: Stampede protection is broken.
- Fixed GH #328: Segfault in apcu_key_info() if APCu is disabled.
- Generally make the behavior of functions if APCu is disabled more consistent.
- Fixed PHP bug #72980: Empty strings are now consistently allowed as cache
  keys.
- Optimized apcu_key_info() and apcu_cache_info() by using interned strings.
- Fix build against PHP master (PHP 7.4).
- Many changes to internal C APIs.

5.1.12
- gh#307: Fix 'Timout' sort option (apc.php).
- gh#308: Keep search parameter on cache entry detail link (apc.php).
- Fix --enable-apcu-clear-signal support.
- Show entries with expired global TTL in APCuIterator.
- Respect TTL when calculating APCuIterator totals.
- The per-entry TTL now always takes precedence over the global TTL.
- The global TTL is now always relative to the access time.
- apcu_inc() and apcu_dec() no longer update hard-expired entries. Instead a
  new entry is created.
- Added optional $ttl argument to apcu_inc() and apcu_dec(), used when creating
  a new entry.
- PHP bug #76145: Fix use of APCu inside Serializer::(un)serialize().
- gh#304: If apcu_cas() is used on a non-existing entry, don't insert it.
- gh#295: Improve APCuIterator performance by using PCRE JIT and preallocating
  key strings.
- Reduce the memory overhead of cache entries.
- Prevent potential memory corruption in the cache slam defense implementation.
- Ensure cache entry references are released on bailout during unserialization.
- Make support for atomic operations a hard requirement for building APCu.
- Check write-lock acquisition for failure, to help debugging deadlock
  situations.
- Make sure apcu_inc/dec are atomic when working on a non-existing entry.
- Many changes to internal C APIs.

5.1.11
- fix gh#246 apcu_entry hangs
- fix gh#259 deadlock in apcu_store
- fix gh#281 undefined variable in apc.php
- fix handling of fatal errors in apcu_entry
- check string lengths when looking up keys
- many internal C APIs changed