build: Don't export GResource-related symbols with newer GLib
This makes the desired behaviour explicit, and matches the behaviour
seen with Meson, where external : false
is the default.
Before glib!1468 (merged), not passing --internal to the resource compiler meant "no special export attribute, do what you would normally do", so these symbols were not exported due to our global use of -fvisibility=hidden.
However, since glib!1468 (merged), not passing --internal to the resource compiler results in the symbols being decorated with G_MODULE_EXPORT, which overrides -fvisibility=hidden. This was necessary because Windows DLLs normally behave a bit like the equivalent of ELF libraries with -fvisibility=hidden.
Resolves: #2919 (closed)