Add extern “C” block to header files

[MarkoPura]

By adding an extern "C" block to header files, we avoid linking problems when building C++ applications that come from name mangling since C++ supports function overloading.

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Code Coverage

Type	Coverage
lines	56.9% (1273 of 2238 lines)
functions	64.9% (126 of 194 functions)

[mniestroj]

I get why include/golioth/*.h files need that. But how about src/*.h? Those are private and should not be built with C++ compiler.

[MarkoPura]

I agree @mniestroj and was debating with myself whether to add it to src/*.h files or not. I've left it there in the end since it "doesn't hurt", but I can always remove it.

[sam-golioth]

Is this also required in the private headers? Or only in the public headers? These headers in src/ aren't available to user applications, so they will only be included by C files, not C++.

[MarkoPura]

I'll remove it from the private headers in src/

[mniestroj]

I agree @mniestroj and was debating with myself whether to add it to src/*.h files or not. I've left it there in the end since it "doesn't hurt", but I can always remove it.

IMO it does hurt :) Any additional code that is not used is a dead code. Dead code not tested and potentially not working. Even if that is only so small, #ifdef __cplusplus will never be true.

So I opt for removing it from private headers.

[MarkoPura]

@sam-golioth, @mniestroj removed from the src/ folder!

[sam-golioth]

Thanks @MarkoPura!