build: Use non-deprecated form of warning flags

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-3.

diff --git a/src/Makefile.am b/src/Makefile.am
index 32a51ac..d10f223 100644 (file)
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -78,7 +78,7 @@ BOBJS += main.cmo
 
 XOBJS = $(BOBJS:.cmo=.cmx)
 
-OCAMLFLAGS = -g -warn-error CDEFLMPSUVYZX-3 -ccopt '@CFLAGS@'
+OCAMLFLAGS = -g -warn-error +C+D+E+F+L+M+P+S+U+V+Y+Z+X-3 -ccopt '@CFLAGS@'
 OCAMLLIBS =
 
 bin_PROGRAMS = virt-top