DSO Linking does not allow undefined symbols

[jgriffiths]

This needs to be changed to allow undefined symbols, at least when building a python module, since the preferred build option (and only option for manylinux/pypi) lets the interpreter provide the python symbols.

Given that the default is to build a library that can be a module or a shared library, I suggest we add a manylinux build option for this to build a "python module only" DSO, as per the wally manylinux changes.