btrfs-progs: fix cross

pkgs/tools/filesystems/btrfs-progs/default.nix

@@ -1,5 +1,5 @@
 { stdenv, fetchurl, fetchpatch, pkgconfig, attr, acl, zlib, libuuid, e2fsprogs, lzo
-, asciidoc, xmlto, docbook_xml_dtd_45, docbook_xsl, libxslt, zstd, python3, python3Packages
+, asciidoc, xmlto, docbook_xml_dtd_45, docbook_xsl, libxslt, zstd, python3
 }:
 
 stdenv.mkDerivation rec {
@@ -12,10 +12,17 @@ stdenv.mkDerivation rec {
   };
 
   nativeBuildInputs = [
-    pkgconfig asciidoc xmlto docbook_xml_dtd_45 docbook_xsl libxslt python3 python3Packages.setuptools
+    pkgconfig asciidoc xmlto docbook_xml_dtd_45 docbook_xsl libxslt
+    python3 python3.pkgs.setuptools
   ];
 
-  buildInputs = [ attr acl zlib libuuid e2fsprogs lzo zstd ];
+  buildInputs = [ attr acl zlib libuuid e2fsprogs lzo zstd python3 ];
+
+  # for python cross-compiling
+  _PYTHON_HOST_PLATFORM = stdenv.hostPlatform.config;
+  postConfigure = ''
+    export LDSHARED="$LD -shared"
+  '';
 
   # gcc bug with -O1 on ARM with gcc 4.8
   # This should be fine on all platforms so apply universally