rustPackages: make rustc-dev optional

This was only introduced in 1.40.0 and doesn't work on older versions.

    thread 'main' panicked at 'Error: no rules matched rustc-dev.', src/bootstrap/builder.rs:231:21
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
    failed to run: /build/rustc-1.38.0-src/build/bootstrap/debug/bootstrap dist rustc-dev

pkgs/development/compilers/rust/rustc.nix

@@ -4,6 +4,7 @@
 , pkgconfig, openssl
 , which, libffi
 , withBundledLLVM ? false
+, enableRustcDev ? true
 , version
 , sha256
 }:
@@ -132,12 +133,13 @@ in stdenv.mkDerivation rec {
   outputs = [ "out" "man" "doc" ];
   setOutputFlags = false;
 
-  postInstall = ''
+  postInstall = stdenv.lib.optionalString enableRustcDev ''
     # install rustc-dev components. Necessary to build rls, clippy...
     python x.py dist rustc-dev
     tar xf build/dist/rustc-dev*tar.gz
     cp -r rustc-dev*/rustc-dev*/lib/* $out/lib/
 
+  '' + ''
     # remove references to llvm-config in lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/librustc_codegen_llvm-llvm.so
     # and thus a transitive dependency on ncurses
     find $out/lib -name "*.so" -type f -exec remove-references-to -t ${llvmShared} '{}' '+'