I ran into this problem on AIX 7.1 with GNU Tar, which
enables visibility of getc_unlocked etc. after testing
whether they’re declared, causing argp-namefrob.h and
unlocked-io.h to redefine the macros.
* lib/argp-namefrob.h, lib/unlocked-io.h:
(clearerr_unlocked, feof_unlocked ferror_unlocked)
(fflush_unlocked, fgets_unlocked, fputc_unlocked, fputs_unlocked)
(fread_unlocked, fwrite_unlocked, getc_unlocked, getchar_unlocked)
(putc_unlocked, putchar_unlocked): Do not define if already defined.