better check for sysctl() availability

As it was mentioned before some Linux ports are dropping sysctl support.
This is intentional move and in future versions of Linux kernel this system
call will eventually go away. As a good alternative one may and should use
procfs (/proc/sys).

Currently we chech if corresponding header "sys/sysctl.h" exists.
But the problem happens if libc in use still installs "sys/sysctl.h" header
while provides no implementation for it.

This is exactly the case with uClibc and probably other implementations of libc.
But if we check for existing "sysctl" symbol in libc we're golden at least in
compile-time because libc provides some stub at least.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>