build: fix the version number when build via opam

lib/generate-version.sh

@@ -3,7 +3,7 @@
 set -e
 
 SHA=$(git log -1 --pretty=format:"%h" || echo "opam")
-VERSION=$(git describe --tags || basename `pwd` | cut -d '.' -f2-4)
+VERSION=$(git describe --tags || ls ../../../../ | grep lustre-v6  | cut -d '.' -f2-4)
 BRANCH=$(git branch | grep "*" | cut -d ' ' -f 2 || basename `pwd` | echo "opam")
 
 echo "(** Automatically generated from lib/generate-version.sh *) " > lv6version.ml