Set BUILDING_WITH_NINJA flag when parsing makefiles for ninja build

Distinguish between parsing makefiles in make for a pre-ninja build,
parsing makefiles in kati for a ninja build, and parsing makefiles in
make for a non-ninja build by leaving USE_NINJA=true enabled for the
kati pass but also setting BUILDING_WITH_NINJA.

Change-Id: I63f1ce1efe08cd86fdf41f8051901f720b38cfaf
diff --git a/core/ninja.mk b/core/ninja.mk
index 2d0af34..b590627 100644
--- a/core/ninja.mk
+++ b/core/ninja.mk
@@ -136,7 +136,7 @@
 .INTERMEDIATE: kati.intermediate
 kati.intermediate: $(KATI) $(MAKEPARALLEL)
 	@echo Running kati to generate build$(KATI_NINJA_SUFFIX).ninja...
-	+$(hide) $(KATI_MAKEPARALLEL) $(KATI) --ninja --ninja_dir=$(PRODUCT_OUT) --ninja_suffix=$(KATI_NINJA_SUFFIX) --regen --ignore_dirty=$(OUT_DIR)/% --ignore_optional_include=$(OUT_DIR)/%.P --detect_android_echo --use_find_emulator -f build/core/main.mk $(or $(KATI_TARGETS),--gen_all_phony_targets) USE_NINJA=false
+	+$(hide) $(KATI_MAKEPARALLEL) $(KATI) --ninja --ninja_dir=$(PRODUCT_OUT) --ninja_suffix=$(KATI_NINJA_SUFFIX) --regen --ignore_dirty=$(OUT_DIR)/% --ignore_optional_include=$(OUT_DIR)/%.P --detect_android_echo --use_find_emulator -f build/core/main.mk $(or $(KATI_TARGETS),--gen_all_phony_targets) BUILDING_WITH_NINJA=true
 
 KATI_CXX := $(CLANG_CXX) $(CLANG_HOST_GLOBAL_CFLAGS) $(CLANG_HOST_GLOBAL_CPPFLAGS)
 KATI_LD := $(CLANG_CXX) $(CLANG_HOST_GLOBAL_LDFLAGS)