> It actually doesn't know anything at all about how to build C, C++, or any other kind of code.

I guess it depends on how you define "know", but there are implicit rules.

    $ cat foo.c
    #include <stdio.h>
    int main() {
      printf("Hello\n");
      return 0;
    }
    $ cat Makefile
    foo: foo.c
    $ make
    cc -O2 -pipe    foo.c  -o foo
    $ ./foo
    Hello