Honor configured shell source fallback

ober

77fb1429dc5156dbf9e7319e8bd6567f4e47111e

diff --git a/Makefile b/Makefile
index 7667704..b445cfd 100644
--- a/Makefile
+++ b/Makefile
@@ -225,7 +225,7 @@ jerboa: ensure-jerboa-tools ensure-jerboa-shell
 	done
 	@rm -f jsh-generated.ss jsh-generated.so jsh-generated.wpo jsh.boot jsh-libs.boot
 	@echo "=== Translating .ss → .sls via jerbuild (jerboa transpiler) ==="
-	@echo "    Root overrides take priority; jerboa-shell/ used as fallback."
+	@echo "    Root overrides take priority; $(JERBOA_SHELL_DIR)/ used as fallback."
 	@rm -rf jsh-src && mkdir -p jsh-src/jsh
 	@for m in ast registry macros pregexp-compat util environment lexer arithmetic glob fuzzy \
 	          history recorder player parser functions signals expander \
@@ -233,8 +233,8 @@ jerboa: ensure-jerboa-tools ensure-jerboa-shell
 	          prompt procwatch lineedit fzf script startup main; do \
 	  if [ -f "$$m.ss" ]; then \
 	    cp "$$m.ss" "jsh-src/jsh/$$m.ss"; \
-	  elif [ -f "jerboa-shell/$$m.ss" ]; then \
-	    cp "jerboa-shell/$$m.ss" "jsh-src/jsh/$$m.ss"; \
+	  elif [ -f "$(JERBOA_SHELL_DIR)/$$m.ss" ]; then \
+	    cp "$(JERBOA_SHELL_DIR)/$$m.ss" "jsh-src/jsh/$$m.ss"; \
 	  fi; \
 	done
 	$(JERBUILD) exec --libdirs "$(JH)/lib" support/jerbuild.ss jsh-src src --force