[dune] [configure] Allow to set prefix using environment variable.

This is a hack to enable correct OPAM building, the medium-term plan
is to avoid having to set a prefix at configure time but instead using
a set of rules to locate the Coq library.

We use `(env_var ...)` in a dependency rule, which a feature of Dune 1.2

1 files changed, 4 insertions, 0 deletions

diff --git a/coq.opam b/coq.opam
index 2ac98d8946..cd89057598 100644
--- a/coq.opam
+++ b/coq.opam
@@ -15,6 +15,10 @@ depends: [
   "camlp5"
 ]
 
+build-env: [
+  [ COQ_CONFIGURE_PREFIX = "%{prefix}" ]
+]
+
 build: [
   [ "dune" "build" "@vodeps" ]
   [ "dune" "exec" "coq_dune" "_build/default/.vfiles.d" ]