add #ifdef __cplusplus to headers?

Should something like in https://stackoverflow.com/questions/3789340/combining-c-and-c-how-does-ifdef-cplusplus-work question part inserted to every header when building library ? So library users in cpp don' t need to do it manually.

Found a [post](https://www.functionalnoise.com/pages/2022-07-21-embedding/#interlude_name_mangling) that also took sometime to figure this out. For modern cpp programmers who don' t use C libraries directly often. This could be easilly forgotten.

Start by inspecting how PackageCompiler.jl exposes library headers during a build, then compare the requested C++ linkage guards with the linked Stack Overflow explanation. Determine which headers require the guards and verify that a C++ consumer can include them without adding manual wrappers.

Written by the indexing model from the issue text.