How to install HepMC on your laptop (Linux/Mac OSX) and link to Pythia

As of Pythia 8.175 support for older HepMC versions have been dropped,
so you have to use HepMC version 2.06 (or later), Here is an example 
for HepMC 2.06.08 that works for me, but it comes with no guarantee.

1) Create a directory where you want to do the installation, e.g.
     mkdir hepmc
and move to it
     cd hepmc

2) Open
     http://lcgapp.cern.ch/project/simu/HepMC/download 
in a browser, scroll down to "HepMC 2.06.08" and  pick the version relevant 
for your platform from the list to the right. 

3) In the pop-up menu pick "Save to file" and then the above hepmc directory.
You should now have the appropriate version in that directory,
e.g. HepMC-2.06.08-x86_64-mac106-gcc42-opt.tar.gz on a current Mac.

4) Unpack with
    tar -xzf file.tar.gz
with "file" the name of the downloaded file. This will create a new directory, 
e.g. x86_64-mac106-gcc42-opt/, where ready-to-use libraries can be found.

5) Now move to your pythia8175 (or later) directory and configure with
     ./configure --with-hepmc=mypath/hepmc/newdir --with-hepmcversion=2.06.08
   where "mypath" is the absolute path to the hepmc directory (use pwd if 
   in doubt) and "newdir" is the name of the new directory created in step 4.
   (Re)compile with "make" (or "gmake") as usual.

6) Move to the examples subdirectory and do
     source config.sh
     source config.csh
Only one of them will work, depending on which shell you use, but the other
should be harmless.

7) You should now be able to make and run e.g. main41 to produce HepMC 
output files.