Add enabled_if to Apron cram tests (PR #1500)

Author: sim642

tests/regression/46-apron2/dune

@@ -10,4 +10,6 @@
  (action (chdir ../../.. (run %{update_suite} group apron2 -q))))
 
 (cram
+ (alias runaprontest)
+ (enabled_if %{lib-available:apron})
  (deps (glob_files *.c)))