In OCaml 4.13:
Alert ocaml_deprecated_cli: Setting a warning with a sequence of lowercase or uppercase letters,
like 'CDEFLMPSUVYZX', is deprecated.
Use the equivalent signed form: +C+D+E+F+L+M+P+S+U+V+Y+Z+X+52-3.
])
dnl Substitute OCaml flags and packages.
-AC_SUBST([OCAMLFLAGS], ["-g -safe-string -warn-error CDEFLMPSUVYZX+52-3"])
+AC_SUBST([OCAMLFLAGS], ["-g -safe-string -warn-error +C+D+E+F+L+M+P+S+U+V+Y+Z+X+52-3"])
AC_SUBST([OCAMLPACKAGES], ["-package str,unix,threads -thread"])
dnl Produce output files.