add preprocessor protection

4 jobs for feature/more_file_readers in 52 minutes and 50 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #20254
dune-2.6

00:26:18

passed #20255
dune-2.7

00:33:12

passed #20256
dune-git

00:33:36

passed #20257
dune-git-full

00:42:43