Add easyblock for libint2

Second attempt ...

Original PR (which targeted `master` in error), was #235.

This is a tiny easyblock. I've tested it on several occasions. A corresponding pull request for the easyconfigs was just created.

It seems a bit silly to make an easyblock for something minor like this. Would it not be better to allow for a bit extra functionality in the easyconfigs, such that this can be take care of with a more local solution? For example, I've wondered from the beginning why easyconfigs are not implemented as subclasses of easyblocks. That would (intentionally) blur the lines between what is an easyblock and what is an easyconfig.