reverse the visibility.h files assumptions that we don't build shared libs by default

This avoids having to define define LIBFOO_DLL=1 all over the place. If we ever go with static libs we will
need to define LIBFOO_STATIC=1 but hopefully in some central location like the top level wscript.

Oh, and I also dropped support for gcc older than version 4.x because ardour will already not build
on such an old version.