Bug#770819: libexplain: FTBFS on arm64

[Edmund Grimley Evans <edmund.grimley.evans@gmail.com>]

Source: libexplain
Version: 1.4.D001-2

It failed to build on arm64:

http://buildd.debian.org/status/package.php?p=libexplain&suite=sid

The error was:

libexplain/buffer/enfile.c: In function 'get_maxfile':
libexplain/buffer/enfile.c:74:21: error: 'SYS__sysctl' undeclared
(first use in this function)
         if (syscall(SYS__sysctl, &args) >= 0)
                     ^

As noted in the source, SYS__sysctl is not universally available. I
suggest in libexplain/buffer/enfile.c replacing

#ifdef __linux__

with:

#if defined(__linux__) && defined(SYS__sysctl)

And also making the same replacement in both places in
libexplain/buffer/errno/listen.c.

(Having fixed that, you will probably run into a problem with ustat on
arm64, but that's a separate bug, I think.)