Description:
------------
mcrypt.c appears to be written to use /dev/random for entropy in all cases. As good entropy is a limited resource when obtained from /dev/random, code that makes frequent calls to mcrypt routines that require entropy can block waiting for entropy unexpectedly. 

 PHP_FUNCTION(mcrypt_create_iv) should be modified to permit configuration of /dev/random or /dev/urandom at least at compile time if not at run time.

Reproduce code:
---------------
any code that calls mcrypt routines frequently

Expected result:
----------------
if called frequently enough, will block waiting for entropy.