Use $(OUT_DIR)/ninja_build to mark ninja build.

include $(BUILD_SYSTEM)/config.mk

ifeq ($(USE_NINJA),true)

# Mark this is a ninja build.

$(shell mkdir -p $(OUT_DIR) && touch $(OUT_DIR)/ninja_build)

include build/core/ninja.mk

else # !USE_NINJA

ifeq ($(MAKELEVEL),0)

# Remove ninja build mark if it exists.

$(shell rm -f $(OUT_DIR)/ninja_build)

endif

# Write the build number to a file so it can be read back in

# without changing the command line every time.  Avoids rebuilds