BSD license you code. Put a BSD header in all files in the common library. So long as the license precedes the date of the contract it cannot be argues that this code is their property as it existed before the contract and is already bound by the terms of a license that supersedes the contract.

Remember that BSD gives them rights to do as they please with that code though, they can distribute it sell it, etc. But it is a good license for this type of work, as they have no real (in business terms) restrictions.

You cannot however add to this library of code while on their engagement unless expressly permitted by the contract. It is a good idea to lay out what is in that library in the contract and the kind of items that can be added to the library while on engagement (e.g just bug fixes or generic routines).