netsnmp: Set ac_cv_NETSNMP_CAN_USE_SYSCTL to no

NETSNMP_CAN_USE_SYSCTL is for BSD systems, setting it to yes on
Linux systems breaks compilation when the mibII mib is enabled.

Prior to commit 30bb1bdc ("netsnmp:
bump version") from November 2008, this variable was already set to
no, and was changed to yes by the commit, with no explanation. And the
previous code indeed had a comment saying:

   # We set CAN_USE_SYSCTL to no and use /proc since the sysctl code
   # in this thing is apparently intended for freebsd or some such
   # thing...

[Thomas: improved commit log.]

Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>