DB connection library and dynamic linking

I am working on a piece of code that connects to two different Sybase
databases. One connection is done directly in the code the other is done
through an external library which I do not have control over, I just need to
specify the database name and server in the second scenario.I am using
Sybase Open Client library for both connection and it works fine.

I need to update the application which runs on a Solaris 8 machine to
connect to a MSSQL server instead of Sybase and I have chosen FreeTDS for
that scenario.
An earlier trial using Sybase ct-lib to connect to MSSQL failed, eventhough
the TDS version is set to 4.2. FreeTDS contains also a ct-lib
implementation.

Since both the FreeTDS library and Sybase Open Client library has a ct-lib
implementation. I am facing problem of forcing the application to use the
"right" library for connection. What I need is following scenario

FreeTDS (I have the soruce code) -> MSSQL DB connection
Sybase Open Client (external library) -> Sybase DB connection

Is there any way to force to code, compile or link application to choose a
specific library in the code.