Only declare strl{cpy,cat} if they aren't defines.

On some system (e.g. OS X Mavericks), those are defines to compiler
built-ins, thus trying to declare them will fail.
HashSet.h: Replace whatever that was with proper C++
BPlusTree.h: Remove invalid forward declaration.

That forward declaration was actually a backward declaration, as
system_dependencies.h already included Stack.
PackageInfoParser.cpp: Add missing include for strtoll.
SnifferRules.cpp: Make the arguments for operator< const.

It is used on const versions of sniffer_rule and thus it needs to be
declared with const.