* Header files that are used by a compilation must be declared

explicitly.  If you forget a dependency, it's simply not visible to
  the compiler, and so the compilation fails.  This is a big plus over
  conventional Make.

examples/simple-header/hello.c

@@ -0,0 +1,9 @@
+#include <stdio.h>
+
+#include "hello.h"
+
+int main(int argc, char * * argv)
+{
+    printf("Hello " WHAT "\n");
+    return 0;
+}