Marek Vavrusa authored Jan 10, 2016 and Ondřej Surý committed Jan 11, 2016

the Requires list contained a trailing comma if liburcu wasn't present,
pkg-config as of 0.29 treats this as an error, resulting in failed
package --cflags and --atleast-version lookups

this change treats libdnssec as mandatory, and extras must be prefixed with
comma inside the expanded variables