require setting the OCaml version

buildjob

@@ -4,8 +4,8 @@ set -eo pipefail
 . ci/ansi-colors.sh
 
 ## This runs a default build job.  The following variables are noteworthy:
-## - $OCAML: The OCaml version to use. Defaults to `ocaml-system`. This MUST
-##   start with a package name, e.g. `ocaml-base-compiler.4.07.0`.
+## - $OCAML: The OCaml version to use. This MUST start with a package name, e.g.
+##   `ocaml-base-compiler.4.07.0`.
 ## - $OPAM_PINS: Space-separated list of packages to pin in opam.
 ##   Each package can be:
 ##   - a group of three list elements: `PACKAGE KIND TARGET`.
@@ -38,11 +38,13 @@ if [[ "$CI_RUNNER_DESCRIPTION" == "coop-timing" && -n "$TIMING_AD_HOC_ID" ]]; th
     status "[buildjob] Ad-hoc timing run with ID $TIMING_AD_HOC_ID"
 fi
 
-OCAML=${OCAML:-ocaml-base-compiler.4.08.1}
 MAKE_TARGET=${MAKE_TARGET:-all}
 TIMING_PROJECT=${TIMING_PROJECT:-$CI_PROJECT_NAME}
 
 # Environment sanity checks
+if [[ -z "$OCAML" ]]; then
+    panic "[buildjob] The OCAML variable needs to be set to the desired OCaml version (e.g. ocaml-base-compiler.4.07.0)"
+fi
 if [[ -n "$VALIDATE" ]]; then
     panic "[buildjob] Do not use the VALIDATE variable, it has been replaced by CI_COQCHK"
 fi