rPackages.ragg: fix build on darwin

Without this, x86_64-darwin build fails with:

> ld: library not found for -lLerc

Seems not needed on x86_64-linux, as that succeeds.

pkgs/development/r-modules/default.nix

@@ -679,7 +679,7 @@ let
     DropletUtils = [ pkgs.zlib.dev ];
     RMariaDB = [ pkgs.libmysqlclient.dev ];
     ijtiff = [ pkgs.libtiff ];
-    ragg = with pkgs; [ freetype.dev libpng.dev libtiff.dev zlib.dev libjpeg.dev bzip2.dev ];
+    ragg = with pkgs; [ freetype.dev libpng.dev libtiff.dev zlib.dev libjpeg.dev bzip2.dev ] ++ lib.optional stdenv.isDarwin lerc.dev;
     qqconf = [ pkgs.fftw.dev ];
     vapour = with pkgs; [ proj.dev gdal ];
   };