flambda-backend: Fix incorrect use of AC_ARG_ENABLE (#2419)

Author: stedolan

Makefile.config.in

@@ -245,11 +245,7 @@ POLL_INSERTION=@poll_insertion@
 DUNE=@dune@
 
 # Selection of OCaml 4.x or 5.x runtime
-ifeq "@enable_runtime5@" "yes"
-  RUNTIME_SUFFIX=
-else
-  RUNTIME_SUFFIX=4
-endif
+RUNTIME_SUFFIX=@runtime_suffix@
 RUNTIME_DIR=runtime$(RUNTIME_SUFFIX)
 
 # Deprecated variables

configure

@@ -93,10 +93,14 @@ AC_CONFIG_AUX_DIR([build-aux])
 AC_ARG_ENABLE([runtime5],
   [AS_HELP_STRING([--enable-runtime5],
     [Use the OCaml 5 runtime])])
+AS_IF([test x"$enable_runtime5" = xyes],
+      [runtime_suffix=],
+      [runtime_suffix=4])
 
 ## Output variables
 
 AC_SUBST([enable_runtime5])
+AC_SUBST([runtime_suffix])
 AC_SUBST([CONFIGURE_ARGS])
 AC_SUBST([native_compiler])
 AC_SUBST([default_build_target])