use go-generate to handle ROOT5/ROOT6

[sbinet]

right now, all the murky details of deciding against which ROOT version we are building (root-5 or root-6) is performed inside the Makefile (via root-config --version) which then pass -tags=root5/root6 to the go commands (install, test, clean).

consider whether it wouldn't be easier to handle this via go generate.

one would put all the logic inside a build.go file which would generate the few files with different handling wrt root-5 and root-6, namely: the dict generation of the go EDM, some cgo flags, enabling reflex or not, and the user-defined types passed to genROOTDict.

the benefit would be a simpler command invokation:

sh> go get -d github.com/go-hep/croot
sh> cd github.com/go-hep/croot
sh> go generate
sh> go install ./...
sh> go test ./...

(of course, this could then be folded into a new simpler Makefile)