C compiles, C++ doesn't

Instead of renaming the file try adding a new file (e.g. test.cpp). Add some C++ code to make sure it really compiles as C++. Then get rid of hello_world.c and add the main() function to your new C++ file.

I think the problem is that once a rule for hello_world.o has been established it's linked to hello_world.c. You may be able to add hello_world.cpp back in after deleting hello_world.c but it's not likely you even want a source file with that name.

Andrew