scid-vs-pc: Set meta platforms to linux

Darwin spits the following error:

  Location of "tk.h": not found
    Location of Tcl 8.6 library: /nix/store/zr4n0fdb3nsdp80vff9mihks22w1g8jm-tcl-8.6.6/lib
    Location of Tk 8.6 library: /nix/store/bwlx46zncj2c6bv2sn75ai0wgzc89d93-tk-8.6.6/lib
    Location of X11 library: /nix/store/093lzjvm2r27rprpqxvyrgqvfp1k0dlf-libX11-1.6.5/lib/
    Checking if your system already has zlib installed: no.

Error: compiler g++ does not support C++0x (C++11).