Tidy for new release

Allow cylc to run from a raw source tree a per github tagged downloads: version string comes from ```git describe``` when in a repo; and from a VERSION file created by the ```make``` process in a raw source tree.  A new top-level Makefile does this, and then drops into ```doc``` to generate the User Guides (and this no longer fails if the user has not installed either or both of ```pdflatex``` and ```htlatex```).  Also proper incremental make for the command help files. See commit logs for a few extra details. 

Passes tests (these changes touch ```lib/cylc``` only minimally).

Once merged I can finally tag 5.0.0 and allow non-git users to download new versions of cylc.