planarity source files fail to compile with `-std=gnu23`

[alejo7797]

GCC 15 changes the default C language version to -std=gnu23, which causes the planarity extension to not build.

The declarations in this file are an issue as per this change between standards.

This is not a big problem as one can simply pass -std=gnu17 but it would be nice to update the planarity source tree.

I was able to build and test Spherogram successfully (on Linux) with GCC 15 by replacing planarity_src/c with the c/graphLib directory from upstream, plus some tiny changes to setup.py and MANIFEST.in.

Happy to open a PR with my changes if that would be welcome, unless there were already plans to update the planarity_src directory.

[culler]

We did not have plans to upgrade the planarity source, but it sounds like a good idea to me (as long as Spherogram continues to work). So it would be great to have your PR.