Add Meson Build System Support to ddX

[lukaswittmann]

We are currently working on the implementation of ddX into our tblite and xTB program packages.
As the meson build system is our main workhorse for compiling, we would need this to be available for ddX.

Is it planned in the near future to incorporate meson? If not, we will look into implementing it ourselves via a PR.

CC: @robin-dahl

[lukaswittmann]

We are currently already implementing it, it can be found in my fork.

[mnottoli]

No, it was not a plan for the immediate future. So thanks for working on it. Please let me know if you need any assistance.

[lukaswittmann]

A pull request is now open (#157) that implements the Meson build system for ddX.