Low: build: add --without snmp flag to RPM spec

SNMP traps only work from net-snmp 5.4 forward. Require net-snmp 5.4
or later and for being able to build on platforms where that release
is not available, add a --without snmp conditional build flag.

Mercurial revision: 3f4e591e647a6dc9ca02dcf7feb8c85a882bfa5c