django: add a django_gis variant which patches django to properly point at its gis libs but therefore also has them as dependencies (gdal for one can be quite big)

Conflicts:
pkgs/top-level/python-packages.nix

pkgs/development/python-modules/django/1.7.7-gis-libs.template.patch

···
+diff --git a/django/contrib/gis/gdal/libgdal.py b/django/contrib/gis/gdal/libgdal.py
+--- a/django/contrib/gis/gdal/libgdal.py
++++ b/django/contrib/gis/gdal/libgdal.py
+@@ -17,7 +17,7 @@ try:
+     lib_path = settings.GDAL_LIBRARY_PATH
+ except (AttributeError, EnvironmentError,
+         ImportError, ImproperlyConfigured):
+-    lib_path = None
++    lib_path = "@gdal@/lib/libgdal.so"
+ 
+ if lib_path:
+     lib_names = None
+diff --git a/django/contrib/gis/geos/libgeos.py b/django/contrib/gis/geos/libgeos.py
+--- a/django/contrib/gis/geos/libgeos.py
++++ b/django/contrib/gis/geos/libgeos.py
+@@ -23,7 +23,7 @@ try:
+     lib_path = settings.GEOS_LIBRARY_PATH
+ except (AttributeError, EnvironmentError,
+         ImportError, ImproperlyConfigured):
+-    lib_path = None
++    lib_path = "@geos@/lib/libgeos_c.so"
+ 
+ # Setting the appropriate names for the GEOS-C library.
+ if lib_path:

pkgs/top-level/python-packages.nix

···
 
   django = self.django_1_7;
 
+  django_gis = self.django.override rec {
+    patches = [
+      (pkgs.substituteAll {
+        src = ../development/python-modules/django/1.7.7-gis-libs.template.patch;
+        geos = pkgs.geos;
+        gdal = pkgs.gdal;
+      })
+    ];
+  };
+
   django_1_8 = buildPythonPackage rec {
     name = "Django-${version}";
     version = "1.8.4";