Every dynamic extension must be distributed under a license that is compatible with the GNU GPL (see GNU General Public License).
In order for the extension to tell gawk that it is
properly licensed, the extension must define the global symbol
plugin_is_GPL_compatible. If this symbol does not exist,
gawk 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:
int plugin_is_GPL_compatible;