According to the SUSv3, ftruncate(...) routine shall be visible when including
unistd.h; but it is not when _POSIX_C_SOURCE only is defined (no XSI features).

Sample code:
----------------
#define _POSIX_C_SOURCE 200112L
#include <unistd.h>
#include <sys/types.h>
typedef int (*ftruncate_test)(int fildes, off_t length);
int dummyfcn (void)
{
        ftruncate_test dummyvar;
        dummyvar = ftruncate;
        return 0;
}