Use configuration-nix to include freetds dependency of the odbc package (#1000)

I think this is more correct: other package that depend on the system
odbc package may not need freetds, the problem is that the Haskell odbc
package doesn't declare its dependency on freetds.

Author: michaelpj

lib/system-nixpkgs-map.nix

@@ -73,10 +73,9 @@ with pkgs;
   boost_wave = [ boost ];
   boost_wserialization = [ boost ];
   tensorflow = [ libtensorflow ];
-  # odbc package requires both freetds (https://github.com/fpco/odbc/commit/9457377089dbe84d8c329560fa1bc1a2797c821d)
-  # and unixODBC packages to be installed in order to successfully
+  # odbc package requires unixODBC packages to be installed in order to successfully
   # compile C sources (https://github.com/fpco/odbc/blob/master/cbits/odbc.c)
-  odbc = [ freetds unixODBC ];
+  odbc = [ unixODBC ];
   opencv = [ opencv3 ];
   icuuc = [ icu ];
   icui18n = [ icu ];

modules/configuration-nix.nix

@@ -7,7 +7,7 @@
   # terminfo doesn't list libtinfo in its cabal file. We could ignore
   # this if we used the terminfo shipped with GHC, but this package is
   # reinstallable so we'd rather have it defined in the plan.
-  packages.terminfo.components.library.libs = [pkgs.ncurses];
+  packages.terminfo.components.library.libs = [ pkgs.ncurses ];
 
   # The `extra-libraries` field in `X11.cabal` does not include Xss and Xinerama
   # see https://github.com/input-output-hk/haskell.nix/pull/988
@@ -16,6 +16,10 @@
     pkgs.xorg.libXinerama
   ];
 
+  # odbc needs this package to provide odbcss.h on Linux and macOS, see
+  # https://github.com/fpco/odbc#common-issues
+  packages.odbc.components.library.libs = [ pkgs.freetds ];
+
   # These packages have `license: LGPL` in their .cabal file, but
   # do not specify the version.  Setting the version here on
   # examination of the license files included in the packages.