Add ability to use self-compiled x13as on Mac
When I compile Dynare from source on either Linux or Mac, I also compile x13as from source and put it somewhere in my path, e.g.
~/.local/bin/x13as. For Linux we have the ability to get the path to
x13as by running
which; I don't see a reason why this should not be also useful on a Mac? So this MR does this.
Should also go into 4.7.