Remove '#ifdef __cplusplus extern "C"' sections from C sources

[saxbophone]

Chunks of code like this:

#ifdef __cplusplus
extern "C"{
#endif

are useful in C header files in allowing C++ programs to use C functions, by specifying those functions with C linkage.

However, although this is tolerable when it comes to headers, it shouldn't be used in source files. My source files aren't written with the intention of the code in them being 100% compatible with C++, so I should not put anyone under any illusion that they can be compiled with a C++ compiler (even if that might work).

TL; DR; Make it clear that the library intends to be used by C++ programs, but that the library itself should still be compiled as C.