Make lib usable with C++

[jwupf]

Add 'extern "C"' to function definitions to inhibit name mangling if used in C++ sources.
Closes #63.

Checklist

Thanks a lot for your contribution!
Please tick off the following:

• Tests run successfully (i.e. make test)
  • using the CC65 compiler
  • using the LLVM/Clang compiler
• Doxygen style tags are used for public API
• Source code is properly formatted; run e.g. clang-format -i <file>
• I agree to the License of this repository

I did build with CC65. I can building with Clang and run the tests after setting up my environment for that(if I find the time). But I could use the lib with the changes in my application development environment. I have no such environment setup for this lib, yet.

I did not change comments, so I did not tick that item off.

[mlund]

Thanks, this looks good! A minor thing: there's unrelated code-formatting included in the commits. It seems harmless, but please double-check that it adheres to clang-format, or better change this in in a separate PR.

[jwupf]

That changes come from the commit hooks. I can remove them and add a PR for them.

[mlund]

That changes come from the commit hooks. I can remove them and add a PR for them.

No need; they are pretty small so let's just merge.

[jwupf]

20 minutes playing with git later ... OK ;-)