Add optional Accelerate dependency on Darwin for pkgs.nix
On Mac, building gargantext
requires linking against the
Accelerate framework (completely unrelated to the accelerate
Haskell
package!). This patch provides an extra pkgs.nix
directive that will
kick in only when building on Darwin.
I have tested this on a Linux VM and it seems to go through just fine (i.e., the optional directive won't affect non-Darwin systems).