For what? FPC maintains an Errno for its own kernel queries, and so does libc. IOW to check the error result of FPC functions you need errno, and for the errors of libc functions you need cerrno.
Oldlinux has been long removed. It was a part of the 1.0.x rtl. The cleaned up functionality (including FPC's internal errno) is in unit baseunix. The libc errno is in unit initc.