Symbol visibility issues (Clang)

[ferdonline]

Compiling for mac I see a lot of these warnings

ld: warning: direct access in function 'bind_immutable_module(pybind11::module&)' from file 'CMakeFiles/_morphio.dir/bind_immutable.cpp.o' to global weak symbol 'morphio::SectionBasemorphio::Section::id() const' from file '../../src/libmorphio_static.a(morphology.cpp.o)' means the weak symbol cannot be overridden at runtime. This was likely caused by different translation units being compiled with different visibility settings.

When Werror is active it fails compilation.