pythonPackages.shapely: Fix search for libc

This should fix the search for libc's free method on both linux and
darwin.

pkgs/top-level/python-packages.nix

···
       export LANG="en_US.UTF-8";
     '';
 
-    patchPhase = ''
-      sed -i "s|_lgeos = load_dll('geos_c', fallbacks=.*)|_lgeos = load_dll('geos_c', fallbacks=['${pkgs.geos}/lib/libgeos_c.so'])|" shapely/geos.py
+    patchPhase = let
+      libc = if stdenv.isDarwin then "libc.dylib" else "libc.so.6";
+    in ''
+      sed -i "s|_lgeos = load_dll('geos_c', fallbacks=.*)|_lgeos = load_dll('geos_c', fallbacks=['${pkgs.geos}/lib/libgeos_c.${sharedLibraryExtension}'])|" shapely/geos.py
+      sed -i "s|free = load_dll('c').free|free = load_dll('c', fallbacks=['${stdenv.cc.libc}/lib/${libc}']).free|" shapely/geos.py
     '';
 
     # tests/test_voctorized fails because the vectorized extension is not
···
 
     meta = {
       description = "Geometric objects, predicates, and operations";
+      maintainers = with maintainers; [ knedlsepp ];
       homepage = "https://pypi.python.org/pypi/Shapely/";
     };
   };