Deal with missing lseek64(2) sys call

On OSX, off_t is 64 bits, so can use lseek(2) instead

configure.in

@@ -69,7 +69,14 @@ AC_CHECK_LIB(c,srandomdev)
 dnl Solaris hides nanosleep here
 AC_CHECK_LIB(rt,nanosleep)
 
-AC_CHECK_FUNCS([getpeereid bcopy bzero bcmp])
+AC_CHECK_FUNCS([getpeereid bcopy bzero bcmp lseek64])
+AC_CHECK_TYPES([off64_t], [have_off64_t=1], [have_off64_t=0])
+AC_CHECK_SIZEOF([off_t])
+
+dnl There must be a 64-bit seek(2) system call of some kind
+AS_IF([test \( "x$have_lseek64_t" = "xno" -a "x$ac_cv_sizeof_off_t" != x8 ], [
+    AC_MSG_ERROR([Missing lseek64(2) system call])
+])
 
 AC_CHECK_HEADERS(
     stdio.h \

os.h

@@ -85,6 +85,21 @@ __SERVAL_DNA__OS_INLINE int bcmp(const void *s1, const void *s2, size_t n) {
 }
 #endif
 
+/* If there is no lseek64(2) system call but off_t is 64 bits, then we can use
+ * lseek(2) instead.
+ */
+#ifndef HAVE_LSEEK64
+# if SIZEOF_OFF_T != 8
+#  error "lseek64(2) system call is not available and `sizeof(off_t) is not 8"
+# endif
+# ifndef HAVE_OFF64_T
+typedef off64_t off_t
+__SERVAL_DNA__OS_INLINE off64_t lseek64(int fd, off64_t offset, int whence) {
+    return lseek(fd, offset, whence);
+}
+# endif
+#endif
+
 /* The "e" variants log the error before returning -1.
  */
 int mkdirs(const char *path, mode_t mode);