xfig Merge Request #1: Fix prototypes for modern C (merged)

This MR fixes function callback prototypes such that Xfig will build with modern C compilers. I tested this patch set during the last 2 months in production and didn't notice any misbehavior.

[tkl]

Please, first pull the actual state of master on xfig and rebase the branch prototypes on it
On the xfig repository, if tried to merge into a temporary branch based on master. This gave plenty of "Auto-merging ..." messages, indicating that the commits do not introduce new changes, and some merge conflicts.

git switch master
git branch prototypes
git pull git://git.code.sf.net/u/hamarituc/xfig prototypes

[Mario Haustein]

After rebasing I learnt you already had cherry-picked my commits during February after the discussion in https://sourceforge.net/p/mcj/fig2dev/merge-requests/1/. So this merge request if effectively empty. Sorry for the noise.

[tkl]

Yes, I pulled the changes for xfig from the repository given in https://sourceforge.net/p/mcj/fig2dev/merge-requests/1/. I set the status of this merge request to "merged", since it's content is now contained in xfig.