make: check submodules before configuring

CMakePresets.json includes the gin toolchain presets, so configuring
fails with a bare "File not found" when the submodules are missing or a
git submodule update is in flight. Report the missing path and the fix
instead.

Makefile

@@ -3,7 +3,7 @@
 # bundles into ~/Library/Audio/Plug-Ins and the factory resources into
 # /Library/Audio/Presets (see the PRESET_ROOT note below).
 
-.PHONY: all configure build clean install uninstall uninstall-system pkg
+.PHONY: all configure check-submodules build clean install uninstall uninstall-system pkg
 
 PRESET ?= $(shell \
 	if command -v xcodebuild >/dev/null 2>&1 && xcodebuild -version >/dev/null 2>&1; then \
@@ -21,6 +21,14 @@ CACHE = $(BUILD_DIR)/CMakeCache.txt
 ARTEFACTS = $(BUILD_DIR)/$(PLUGIN)_artefacts/$(CONFIG)
 STAGE = $(BUILD_DIR)/install-stage
 
+SUBMODULE_FILES = modules/gin/ci/toolchains/base.json \
+                  modules/gin/ci/toolchains/xcode.json \
+                  modules/juce/CMakeLists.txt
+
+# CMakePresets.json includes the gin toolchain presets, so configuring dies with
+# a bare "File not found" if the submodules are missing or mid-update.
+SUBMODULE_GUARD = for f in $(SUBMODULE_FILES); do test -e "$$f" || { echo "Missing $$f"; echo "Run: git submodule update --init --recursive"; exit 1; }; done
+
 # Bundles go in the user directory so no sudo is needed and hosts prefer them.
 PLUGIN_ROOT = $(HOME)/Library/Audio/Plug-Ins
 
@@ -34,7 +42,12 @@ all: build
 
 configure: $(CACHE)
 
+check-submodules:
+	@$(SUBMODULE_GUARD)
+	@echo "Submodules look complete."
+
 $(CACHE):
+	@$(SUBMODULE_GUARD)
 	cmake --preset $(PRESET)
 
 build: $(CACHE)