If you confine use of the library to within the walls of your corporation, you do not have to distribute the source (even to your employees), because you are not redistributing (selling or giving away a software product that includes the library) outside of your organization.
The GPL allows you to freely use the code inside a corporation without restrictions, and that includes (by necessity) your ability to prevent your employees (as a matter of company policy) from distributing the source code outside the organization.
From the Gnu Licensing FAQ:
Is making and using multiple copies within one organization or
No, in that case the organization is
just making the copies for itself. As a consequence, a company or
other organization can develop a modified version and install that
version through its own facilities, without giving the staff
permission to release that modified version to outsiders.
However, when the organization transfers copies to other organizations
or individuals, that is distribution. In particular, providing copies
to contractors for use off-site is distribution.