It is desirable to declare standard libraries available on a given system. Putting target declaration in a specific project's Jamfile is not really good, since locations of the libraries can vary between different development machines and then such declarations would need to be duplicated in different projects. The solution is to declare the targets in Boost.Build's site-config.jam configuration file:

project site-config ;
lib zlib : : <name>z ;

Recall that both site-config.jam and user-config.jam are projects, and everything you can do in a Jamfile you can do in those files as well. So, you declare a project id and a target. Now, one can write:

exe hello : hello.cpp /site-config//zlib ;

in any Jamfile.