--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.