BUILD: makefile: rework 51D to split v3/v4

BUILD: makefile: rework 51D to split v3/v4

There are multiple options for 51DEGREES, v3/v4, threading or not,
pattern/trie for v3, vhash for v4, use of libatomic, etc. While the
current rules deal with all of that correctly, it's too difficult to
focus on one version because the two are interleaved for every single
option. Let's just split them into two independent blocks. This removes
some if/endif, and makes the lecture much more straightforward.