It looks like, on arm32 in <features.h>:
  #ifdef  _XOPEN_SOURCE
  # define __USE_XOPEN    1
  # if (_XOPEN_SOURCE - 0) >= 500
  #  define __USE_XOPEN_EXTENDED  1
  /* ... */

And ptsname(), unlockpt(), grantpt() are under:
  #ifdef __USE_XOPEN_EXTENDED
  extern int grantpt (int __fd) __THROW;
  extern int unlockpt (int __fd) __THROW;
  extern char *ptsname (int __fd) __THROW __wur;
  #endif
  /* ... */

Fixes:
pty00.c:50:2: error: implicit declaration of function 'grantpt' [-Werror=implicit-function-declaration]
  grantpt(fdm);
  ^~~~~~~
pty00.c:51:2: error: implicit declaration of function 'unlockpt' [-Werror=implicit-function-declaration]
  unlockpt(fdm);
  ^~~~~~~~
pty00.c:52:14: error: implicit declaration of function 'ptsname' [-Werror=implicit-function-declaration]
  slavename = ptsname(fdm);
              ^~~~~~~
pty00.c:52:12: error: assignment makes pointer from integer without a cast [-Werror=int-conversion]
  slavename = ptsname(fdm);
            ^
Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>