cygwin: asciidoc otherwise tries to install to //nix and fails on cygwin

'//' on cygwin is the beginning of a SAMBA path

pkgs/tools/typesetting/asciidoc/default.nix

@@ -255,6 +255,7 @@ stdenv.mkDerivation rec {
   '';
 
   preInstall = "mkdir -p $out/etc/vim";
+  makeFlags = if stdenv.isCygwin then "DESTDIR=/." else null;
 
   meta = with stdenv.lib; {
     description = "Text-based document generation system";