Mac/Linux builds: eigen versions 3.4.1 and later cause PHD2 build failure

[agalasso]

PHD2 will only build with Eigen v3.4.0 or earlier.

As a workaround on Mac I was able to grab the 3.4.0 homebrew formula

https://raw.githubusercontent.com/Homebrew/homebrew-core/782a31db0c369c75ac2f13f020f492791b09ab89/Formula/e/eigen.rb

then install it using

HOMEBREW_DEVELOPER=true brew install eigen.rb

We should update PHD2 to work with newer versions of Eigen (eigen3 or the latest, v5)

[MattBlack85]

not sure if here works cause eigen 5 brings eigen3 with it, but I have eiegn 5 on my system (Arch both x64 and aarch64). I can see the /usr/include/eigen3 folder brought with that package and passing -DEIGEN_SRC=/usr/include/eigen3 \ works just fine (also the AUR mantainer of PHD2 confirmed it compiles just fine against eigen 5)