Don't compile C source code as C++!

[saxbophone]

Libsxbp can't be compiled by a C++ compiler anyhow, but to make it abundantly clear that it shouldn't be, replace all

#ifdef __cplusplus
extern "C" {
#endif

blocks in non-header C source files with an error macro that is activated if __cplusplus is defined (and that the code is therefore being compiled with a C++ compiler).

I suppose I could just remove those macro sections completely, but seeing that I already have them in there, it could be nice to leave these explicit messages in there.

Closes #226