Specifying an additional library for the gcc linker

Sounds like what you want is in the ld man page:

--allow-shlib-undefined
--no-allow-shlib-undefined
Allows or disallows undefined symbols in shared libraries. This switch is similar to --no-undefined except that it determines the behaviour when the undefined symbols are in a shared library rather than a regular object file. It does not affect how undefined symbols in regular object files are handled.

The default behaviour is to report errors for any undefined symbols referenced in shared libraries if the linker is being used to create an executable, but to allow them if the linker is being used to create a shared library.

The reasons for allowing undefined symbol references in shared libraries specified at link time are that: 
• A shared library specified at link time may not be the same as the one that is available at load time, so the symbol might actually be resolvable at load time.

• There are some operating systems, eg BeOS and HPPA , where undefined symbols in shared libraries are normal. 
The BeOS kernel for example patches shared libraries at load time to select whichever function is most appropriate for the current architecture. This is used, for example, to dynamically select an appropriate memset function.