Neither the configure option nor configure variable to disable linking
against PCRE seem to work anymore, so simply drop both and add a
dependency on libpcre. As net-snmp is unlikely to fit on devices with
small flash anyway, the extra size requirement shouldn't be a problem.
If it is, feel free to submit a patch to fix the broken upstream
behaviour.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>