Revert "Revert "cpython: fix finding headers when cross-compiling extension modules""

Moving the sysconfig file caused spidermonkey_78 to fail to build. We now symlink it instead.

This reverts commit 6a23dde75b.

pkgs/development/python-modules/bootstrapped-pip/default.nix

@@ -45,7 +45,7 @@ stdenv.mkDerivation rec {
     mv wheel* wheel
     # Set up PYTHONPATH. The above folders need to be on PYTHONPATH
     # $out is where we are installing to and takes precedence
-    export PYTHONPATH="$out/${python.sitePackages}:$(pwd)/pip/src:$(pwd)/setuptools:$(pwd)/setuptools/pkg_resources:$(pwd)/wheel"
+    export PYTHONPATH="$out/${python.sitePackages}:$(pwd)/pip/src:$(pwd)/setuptools:$(pwd)/setuptools/pkg_resources:$(pwd)/wheel:$PYTHONPATH"
 
     echo "Building setuptools wheel..."
     pushd setuptools