Visual Studio does not support things like -Wl,export-dynamic, so we
need to export those symbols by using __declspec(dllexport). So, we
decorate these with macros which we define accordingly for this purpose.
https://bugzilla.gnome.org/show_bug.cgi?id=785210
The `-export-dynamic` flag is a libtool-specific flag; since we're not
using libtool with Meson, we should instruct the C compiler to use the
appropriate linker flag instead.
