c-api: turn const global into define

[VDanielEdwards]

Subject

When linking two separately compiled object files, both using the C-API, the linker informed me about a multiply defined symbol. This turned out to be the const global SilKit_LinDataLengthUnknown.

I did grep through the C headers and haven't found another one of those.

Neither the .dll nor the .so export this symbol so no trouble there.

Issue: SILKIT-1654

Developer checklist (address before review)

• Changelog.md updated
• Prepared update for depending repositories
• Documentation updated (public API changes only)
• API docstrings updated (public API changes only)
• Rebase → commit history clean
• Squash and merge → proper PR title

[MariusBgm]

looks good, as this seems to only affect a non-visible symbol.
I just tested your changes on ubuntu and Windows with a modified SilKitDemoLin (added a use of the symbol) executable. it doesn't complain about missing symbols.