Every dynamic extension should define the global symbol
plugin_is_GPL_compatible to assert that it has been licensed under
a GPL-compatible license. If this symbol does not exist,
emits a fatal error and exits when it tries to load your extension.
The declared type of the symbol should be
int. It does not need
to be in any allocated section, though. The code merely asserts that
the symbol exists in the global scope. Something like this is enough: