This is less of a "Today I learned" article, but more like a Unix
trick that took me too long to realize.

Create a new directory with a hello world C program:

```
$ ls
$ cat > hello.c
#include <stdio.h>

int main() {
  puts("Hello, world!");
  return 0;
}
```

💡 The empty or non-existent Makefile is a valid makefile for this C
program:

```
$ make hello                # build "hello"
cc     hello.c   -o hello
$ ./hello
Hello, world!
```