How to generate a SHA1 hash in C++

Having been put in the position of generating password hashes (again), I thought I better write it down for posterity.
You can always write your own SHA1 implementation, but let's be serious, who wants that? Unless you're doing it for academic purposes, it makes no sense to reinvent the wheel when there are so many good implementations out there.

Among all the available libraries I chose to experiment with OpenSSL and boost.