Ex: G_STRUCT_OFFSET(SpiceLinkReply, num_channel_caps) is 172 bytes
when compiled with -mms-bitfield, and 170 bytes without.
GLib/Gtk are compiled with -mms-bitfield, and it is necessary to
compile with the same option for compatibility. The pack pragma
for MINGW32 corrects the structure alignement.
We could use the pack pragma for GNUC unconditionally, that could help
to ensure struct binary compatibility between different compiled
flavours.