Fix warnings

[ryandesign]

This PR contains commits that fix some warnings reported by clang 17:

• issues found by -Wimplicit-function-declaration (fixes build failure on macOS with Xcode 12 or later reported in Make errors — MacOS Catalina, v10.15.7 #20)
• issues found by -Wimplicit-int

I have only tested this on macOS 12. Please test on other supported platforms before merging to verify I haven't inadvertently broken something.

[ryandesign]

More code changes are needed to eliminate other warnings. I didn't want to put too many things into one PR since that can be hard to review, but the next changes will build on these. If you can merge the open PRs, maybe I can find time to work on more fixes. Or if you have objections to the proposed changes let me know.

[Wikinaut]

@ryandesign Thanks for your plenty reports. I will try to catch up as my next project.

I suppose that you only submitted and submit pull reports which fully works [at least] for you, thanks in advance.

PS
You might have noticed, that the project goes back to the late 80ies. I worked hard (behind the curtains and in contact with Golda and Udi) to get the sources free - which is luckily the case since a while.