better check for sysctl() availability

As it was mentioned before some Linux ports are dropping sysctl support.
But the problem happens if libc in use still installs "sys/sysctl.h" header
even if kernel doesn't support sysctl.

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>