Deprecated <sys/sysctl.h> on Linux; harmless

[gperciva]

Linux and glibc marked <sys/sysctrl.h> as deprecated, so we've been treated to warnings like:

/usr/include/aarch64-linux-gnu/sys/sysctl.h:21:2: warning: #warning "The <sys/sysctl.h> header is deprecated and will be removed." [-Wcpp]
   21 | #warning "The <sys/sysctl.h> header is deprecated and will be removed."
      |  ^~~~~~~

This is completely harmless for scrypt, as memlimit.c checks multiple methods of getting the available memory, and uses the lowest one. If autoconf's AC_CHECK_HEADER could take an extra parameter saying "only mark this header as avilable if it compiles without any warnings", I'd gladly enable that, but unfortunately there's no such option [1].

Incidently, the sysctl() function was removed in glibc 2.32 (2020-08-06). The binary symbol is still present, but will always return ENOSYS [2].

[1] https://www.gnu.org/software/autoconf/manual/autoconf-2.61/html_node/Generic-Headers.html
[2] https://sourceware.org/pipermail/libc-announce/2020/000029.html

I'm going to close this issue immediately; it's just here in case somebody wonders about these warnings and searches the github issues.