Allow to customize file name of make dist result.

Fixes https://github.com/JuliaLang/julia/issues/7407
Now it is possible to setup own file name mask in Make.user, for example 
```JULIA_DIST_TARNAME =  julia-$(shell date +'%Y%m%d')-$(JULIA_COMMIT)-$(OS)-$(ARCH)```

This is useful on consequent builds, you can add build date to archive name to be able to sort files.