gdal: Fix darwin build

This fixes:
  error: cannot initialize a variable of type 'GooString *' with an rvalue of type 'const GooString *'
which occurs with recent clang or gcc-7.
See OSGeo/gdal#573

pkgs/development/libraries/gdal/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, unzip, libjpeg, libtiff, zlib
+{ stdenv, fetchurl, fetchpatch, unzip, libjpeg, libtiff, zlib
 , postgresql, mysql, libgeotiff, pythonPackages, proj, geos, openssl
 , libpng, sqlite, libspatialite, poppler, hdf4
 , libiconv
@@ -16,6 +16,16 @@ stdenv.mkDerivation rec {
     sha256 = "18iaamzkn0lipizynvspf3bs5qzgcy36hn6bbi941q8dlfdf8xbg";
   };
 
+  patches = [
+    # fix build with recent Poppler
+    (fetchpatch {
+      url    = "https://github.com/OSGeo/gdal/commit/124f0343436d1267319ac627fc220530091b41ea.diff";
+      stripLen = 2;
+      extraPrefix = "";
+      sha256 = "1v6iiy4cgrdcfas3iva5swh9446pqfjh5p6bcab6y49hyjhpsgfy";
+    })
+  ];
+
   buildInputs = [ unzip libjpeg libtiff libpng proj openssl sqlite
     libspatialite poppler hdf4 ]
   ++ (with pythonPackages; [ python numpy wrapPython ])