try weirder tricks for version generation

.gitignore

@@ -7,3 +7,6 @@ _opam
 *.install
 *.exe
 *.tmp
+src/lib/version.ml
+src/lib/.git_index_path
+src/lib/.git_index.lnk

src/lib/dune

@@ -1,6 +1,19 @@
+(rule
+ (target .git_index_path)
+ (deps (universe) gen_git_index_path.sh)
+ (action
+  (with-stdout-to
+   .git_index_path
+   (run sh gen_git_index_path.sh))))
+
+(rule
+ (target .git_index.lnk)
+ (action
+  (run ln -sf %{read-lines:.git_index_path} %{target})))
+
 (rule
  (target version.ml)
- (deps (universe) gen_version.sh)
+ (deps (file .git_index.lnk) gen_version.sh)
  (action
   (with-stdout-to
    version.ml

src/lib/gen_git_index_path.sh

@@ -0,0 +1,2 @@
+#!/bin/sh
+git rev-parse --git-path index 2>/dev/null || echo /dev/null