libc: Replace a reference to TARGET_BUILD_TYPE with DEBUG_BIONIC_LIBC

We're soon going to stop using TARGET_BUILD_TYPE==debug to debug
native modules.

Signed-off-by: Dave Bort <dbort@android.com>
diff --git a/libc/Android.mk b/libc/Android.mk
index 330802a..333dd1e 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -338,7 +338,7 @@
 		-DINET6 \
 		-I$(LOCAL_PATH)/private \
 
-ifeq ($(TARGET_BUILD_TYPE),debug)
+ifeq ($(strip $(DEBUG_BIONIC_LIBC)),true)
   libc_common_cflags += -DDEBUG
 endif