-STAGE2_CFLAGS += -fsanitize=undefined
-STAGE3_CFLAGS += -fsanitize=undefined
-POSTSTAGE1_LDFLAGS += -fsanitize=undefined -static-libubsan -lpthread \
+STAGE2_CFLAGS += -fsanitize=undefined -DUBSAN_BOOTSTRAP
+STAGE3_CFLAGS += -fsanitize=undefined -DUBSAN_BOOTSTRAP
+POSTSTAGE1_LDFLAGS += -fsanitize=undefined -static-libubsan -DUBSAN_BOOTSTRAP \
+ -B$$r/prev-$(TARGET_SUBDIR)/libsanitizer/ \