Instead of having three separate copies of the function list, use macro
magic to make it so there is only one list that we use in three
different cases.
Apparently in C, if you have `void test();`, it's completely okay to do
`test(2);`. The function will take in the argument, but just discard it
and throw it away. It's like a trash can, and a rude one at that. If you
declare it like `void test(void);`, this is prevented.
This is not a problem in C++ - doing `void test();` and `test(2);` is
guaranteed to result in a compile error (this also means that right now,
at least in all `.cpp` files, nobody is ever calling a void parameter
function with arguments and having their arguments be thrown away).
However, we may not be using C++ in the future, so I just want to lay
down the precedent that if a function takes in no arguments, you must
explicitly declare it as such.
I would've added `-Wstrict-prototypes`, but it produces an annoying
warning message saying it doesn't work in C++ mode if you're compiling
in C++ mode. So it can be added later.
M&P contains network code despite M&P not being a Steam/GOG release (as
Steam/GOG releases are full releases of the game, not
custom-levels-only releases).
While unlocking achievements is already ifdef'd out in M&P, let's remove
the network code entirely to make sure people can't do other shenanigans
with M&P builds, and also to have a smaller binary size.