hash_hmac() undefined function error on new install

Seriously, you should just recompile PHP with ext/hash enabled. Or scream at the person who didn't do it, or change the hosting provider if you're using one ...

Not having the Hash extension will get you in all kinds of trouble, and if you don't have it, chances are there will be a lot more things messed up in your setup. Don't just copy a user-space function for hash_hmac() to work around the problem, it's not worth it.

How to recompile PHP to enable ext/hash? I've googled for it but found nothing

It's enabled by default, just don't disable it.