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.