Against the documentation, at least on Linux systems the const CRYPT_SALT_LENGTH is 2 even when the system is capable of encrypting MD5.
Most likely, this is related to bug #9177.
As stated there, I compiled php (after a make clean; rm config.cache) without openssl support, but 
<?php
printf("%d", CRYPT_SALT_LENGTH);
?>
still emits 2 (but MD5 encryption works fine).
It gets pretty complicated to maintain compatibility with former versions of PHP. This bug also causes compatibility probs when porting DBs with crypt() encrypted passwords from Linux to BSD and vice versa (MD5 ist std on most (all?) BSD platforms.
I'd like to propose the following:
CRYPT_SALT_LENGTH should be set to the longest salt the system is capable of (like it is staded in the docs).
The 4 different consts specifying the salt of a requested encryprion (eg. CRYPT_MD5) should be set to 0 (==not available) or the salt length for this kind/flavour of encryption.