poudriere, pkgng, memcache and php upgrades

[bw.mail.lists]

We use memcache with php. When php gets updated, memcache gets rebuilt 
by poudriere, but pkgng doesn't see it as being different on 'pkg 
upgrade' so it doesn't upgrade the package. Which means that on apache 
restart the error log contains:

PHP Warning:  PHP Startup: Unable to load dynamic library 
'/usr/local/lib/php/20100525/memcache.so' - 
/usr/local/lib/php/20100525/memcache.so: Undefined symbol 
"php_session_create_id" in Unknown on line 0
PHP Warning:  PHP Startup: Unable to load dynamic library 
'/usr/local/lib/php/20100525/memcached.so' - 
/usr/local/lib/php/20100525/memcached.so: Undefined symbol 
"ps_globals" in Unknown on line 0

'pkg upgrade -f' solves the problem, but is there maybe another way?