Revise standalone install script to install all pkgs in same prefix

Summary:
Revise standalone install script to install all the packages (EvtGen + HepMC +
external generators) in the same prefix and make it simpler to specify versions
of the external dependencies

Update README with new CMake options

Cosmetic changes to CMake printout

Allow C++ std to be specified via env var

Look for packages in macOS frameworks after UNIX-style pacakges

Test Plan: Run standalone install script on Linux and macOS

Reviewers: jback, averbyts, kreps

Reviewed By: kreps

Subscribers: #evtgen_admins

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D23