[proposal] Export C functions for use in C++

[sigidagi]

To call C functions from C++ some small changes can be added to neco.h interface. See forked version of neco.h .

I am writing a small C++ wrapper for neco library: necopp . So, in that case it could be useful to link dependencies to direct source instead of forked version of neco library. (Utilizing cmake as a package manager).

[tidwall]

I'd be willing to add C++ support.