[stable/11] error during buildworld on lib/libcasper/libcasper/libcasper_impl.c

Dear All,

I am trying to build world on the stable/11 branch and it fails with
the errors reported below.

Apparently, file /usr/src/lib/libcasper/libcasper/libcasper_impl.c is
not including <unistd.h>

 # uname -a
FreeBSD myhost 11.4-STABLE FreeBSD 11.4-STABLE #1 r369279: Tue Feb 16 10:16:53 CET 2021     root@myhost:/usr/obj/usr/src/sys/GENERIC  amd64

The error is present on both ``heads'' of the SVN and GIT repositories.

Error log:

----8<--------8<--------8<--------8<--------8<--------8<--------8<---------

--- lib/libcasper__L ---                
/usr/src/lib/libcasper/libcasper/libcasper_impl.c:53:13: error: use of undeclared identifier 'STDERR_FILENO'     
        if (*fdp > STDERR_FILENO)

/usr/src/lib/libcasper/libcasper/libcasper_impl.c:60:17: error: use of undeclared identifier 'STDERR_FILENO'                                                    
        while (*fdp <= STDERR_FILENO) {
                       ^                                                        
/usr/src/lib/libcasper/libcasper/libcasper_impl.c:61:9: error: implicit declaration of function 'dup' is invalid in C99 [-Werror,-Wimplicit-function-declaration]                                                                               
                nfd = dup(*fdp);


/usr/src/lib/libcasper/libcasper/libcasper_impl.c:64:7: error: implicit declaration of function 'dup2' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
                if (dup2(nullfd, *fdp) == -1)
                    ^
/usr/src/lib/libcasper/libcasper/libcasper_impl.c:64:7: note: did you mean 'dup'?
/usr/src/lib/libcasper/libcasper/libcasper_impl.c:61:9: note: 'dup' declared here
                nfd = dup(*fdp);
                      ^
/usr/src/lib/libcasper/libcasper/libcasper_impl.c:69:2: error: implicit declaration of function 'close' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        close(nullfd);

----8<--------8<--------8<--------8<--------8<--------8<--------8<---------

Should anyone need any more information, just let me know.

Best regards,
-- 
Arrigo