build: accommodate new syntax-check test

build: accommodate new syntax-check test

* amiga/tailor.c (_expand_args): Change each of three uses of
"exit(20)" to "exit(EXIT_FAILURE)".
* sample/add.c: Include <stdlib.h>.
(main): Use EXIT_FAILURE and EXIT_SUCCESS, not 1 and 0.
* sample/sub.c (main): Likewise.
* sample/zread.c (main): Likewise.