Build warning when not valid work-tree (#19475)

Makefile

@@ -19,8 +19,13 @@ endif
 # Otherwise the [OK], [ERROR] and [WARN] messages won't be displayed correctly
 override SILENT := false
 
+ifeq ($(shell git rev-parse --is-inside-work-tree 2>/dev/null),)
+    export SKIP_GIT := yes
+    export NOT_REPO := yes
+endif
+
 ifdef SKIP_VERSION
-    SKIP_GIT := yes
+    export SKIP_GIT := yes
 endif
 
 ifndef SUB_IS_SILENT
@@ -394,8 +399,11 @@ endef
 	cmp $(ROOT_DIR)/Makefile $(ROOT_DIR)/Makefile >/dev/null 2>&1; if [ $$? -gt 0 ]; then printf "$(MSG_NO_CMP)"; exit 1; fi;
 	# Ensure that $(QMK_BIN) works.
 	if ! $(QMK_BIN) hello 1> /dev/null 2>&1; then printf "$(MSG_PYTHON_MISSING)"; exit 1; fi
-	# Check if the submodules are dirty, and display a warning if they are
+ifdef NOT_REPO
+	printf "$(MSG_NOT_REPO)"
+endif
 ifndef SKIP_GIT
+	# Check if the submodules are dirty, and display a warning if they are
 	if [ ! -e lib/chibios ]; then git submodule sync lib/chibios && git submodule update --depth 50 --init lib/chibios; fi
 	if [ ! -e lib/chibios-contrib ]; then git submodule sync lib/chibios-contrib && git submodule update --depth 50 --init lib/chibios-contrib; fi
 	if [ ! -e lib/lufa ]; then git submodule sync lib/lufa && git submodule update --depth 50 --init lib/lufa; fi

builddefs/message.mk

@@ -64,6 +64,8 @@ MSG_ASSEMBLING = Assembling:
 MSG_CLEANING = Cleaning project:
 MSG_CREATING_LIBRARY = Creating library:
 MSG_GENERATING = Generating:
+MSG_NOT_REPO = $(WARN_COLOR)WARNING:$(NO_COLOR) Target folder is not a git repo, you probably downloaded a zip file instead of cloning.\n\
+Please consider following $(BOLD)https://docs.qmk.fm/\#/newbs_getting_started$(NO_COLOR).\n\n
 MSG_SUBMODULE_DIRTY = $(WARN_COLOR)WARNING:$(NO_COLOR) Some git submodules are out of date or modified.\n\
 Please consider running $(BOLD)qmk git-submodule$(NO_COLOR).\n\n
 MSG_NO_CMP = $(ERROR_COLOR)Error:$(NO_COLOR)$(BOLD) cmp command not found, please install diffutils\n$(NO_COLOR)