Surrounding includes with an 'extern "C"' block is not necessary anymore. Also that made it harder to add any C++ code to some headers, or include headers that have "optional" C++ code like `MEM_guardedalloc.h`. I tested compilation on linux and windows (and got help from @LazyDodo). If this still breaks compilation due to some linker error, the header containing the symbol in question is probably missing an 'extern "C"' block. Differential Revision: https://developer.blender.org/D7653