Fix undefined variable warning for $( ) in Makefiles

Changes

@@ -99,8 +99,8 @@ Working version
 - #9582: Add Array.{find_opt,find_map,split,combine}.
   (Nicolás Ojeda Bär, review by Daniel Bünzli and Gabriel Scherer)
 
-* #10169: Use capitalized module names in the Standard Library prefixing scheme
-  to match Dune, e.g. Stdlib__String instead of Stdlib__string. This is a
+* #10169, #10270: Use capitalized module names in the Standard Library prefixing
+  scheme to match Dune, e.g. Stdlib__String instead of Stdlib__string. This is a
   breaking change only to code which attempted to use the internal names before.
   The Standard Library generated by the Dune rules is now equivalent to the main
   build (the Dune rules still do not generate a distributable compiler).

Makefile.common

@@ -23,6 +23,11 @@ DEPDIR=.dep
 D=d
 MKDIR=mkdir -p
 
+# $(EMPTY) is defined in Makefile.config, but may not have been loaded
+EMPTY :=
+# $(SPACE) contains a single space
+SPACE := $(EMPTY) $(EMPTY)
+
 DESTDIR ?=
 INSTALL_BINDIR := $(DESTDIR)$(BINDIR)
 INSTALL_LIBDIR := $(DESTDIR)$(LIBDIR)

stdlib/Makefile

@@ -239,9 +239,6 @@ clean::
 
 include .depend
 
-EMPTY :=
-SPACE := $(EMPTY) $(EMPTY)
-
 STDLIB_NAMESPACE_MODULES = $(subst $(SPACE),|,$(STDLIB_PREFIXED_MODULES))
 
 .PHONY: depend

stdlib/StdlibModules

@@ -15,6 +15,8 @@
 #*                                                                        *
 #**************************************************************************
 
+# This file must be self-contained.
+
 # This file lists all standard library modules. It is used by:
 # 1. stdlib/Makefile when building stdlib.cma
 # 2. Makefile to expunge the toplevels
@@ -45,12 +47,15 @@ STDLIB_MODULE_BASENAMES=\
 STDLIB_PREFIXED_MODULES=\
   $(filter-out stdlib camlinternal%, $(STDLIB_MODULE_BASENAMES))
 
+define add_stdlib_prefix_first
+$(shell echo $1 | cut -c1 | tr '[:lower:]' '[:upper:]')
+endef
+
 # add stdlib__ as prefix to a module except for internal modules
 # and the stdlib module itself
 define add_stdlib_prefix
   $(or $(filter-out $(STDLIB_PREFIXED_MODULES), $1), \
-    stdlib__$(shell echo $1 | cut -c1 | tr '[:lower:]' '[:upper:]')$\
-            $(shell echo $1 | cut -c2-))
+    stdlib__$(call add_stdlib_prefix_first,$1)$(shell echo $1 | cut -c2-))
 endef
 
 STDLIB_MODULES:=\

tools/ci/actions/runner.sh

@@ -21,6 +21,8 @@ PREFIX=~/local
 MAKE="make $MAKE_ARG"
 SHELL=dash
 
+MAKE_WARN="$MAKE --warn-undefined-variables"
+
 export PATH=$PREFIX/bin:$PATH
 
 Configure () {
@@ -61,8 +63,8 @@ EOF
 }
 
 Build () {
-  $MAKE world.opt
-  $MAKE ocamlnat
+  script --return --command "$MAKE_WARN world.opt" build.log
+  script --return --append --command "$MAKE_WARN ocamlnat" build.log
   echo Ensuring that all names are prefixed in the runtime
   ./tools/check-symbol-names runtime/*.a
 }
@@ -86,6 +88,15 @@ Install () {
 }
 
 Checks () {
+  set +x
+  STATUS=0
+  if grep -Fq ' warning: undefined variable ' build.log; then
+    echo -e '\e[31mERROR\e[0m Undefined Makefile variables detected!'
+    grep -F ' warning: undefined variable ' build.log | sort | uniq
+    STATUS=1
+  fi
+  rm build.log
+  set -x
   if fgrep 'SUPPORTS_SHARED_LIBRARIES=true' Makefile.config &>/dev/null ; then
     echo Check the code examples in the manual
     $MAKE manual-pregen
@@ -106,6 +117,7 @@ Checks () {
   test -z "$(git status --porcelain)"
   # Check that there are no ignored files
   test -z "$(git ls-files --others -i --exclude-standard)"
+  exit $STATUS
 }
 
 CheckManual () {