Define DEBUG macro only when B2G_DEBUG is enabled

Change-Id: Iaa00507ae96a45724848051b8ef05c8a5efb078c
CRs-Fixed: 809836

xpcom.mk

@@ -44,8 +44,9 @@ LOCAL_CFLAGS += -Wno-invalid-offsetof
 # via Android build system.
 LOCAL_CFLAGS += -Wno-error=non-virtual-dtor -Wno-non-virtual-dtor
 
-# Enable DEBUG for -eng builds to activate runtime assertions
-ifneq ($(filter eng, $(TARGET_BUILD_VARIANT)),)
+# Enable DEBUG macro to activate runtime assertions only if B2G_DEBUG is defined
+# to a value other than 0
+ifneq (0,$(or $(strip $(B2G_DEBUG)),0)))
 LOCAL_CFLAGS += -DDEBUG
 endif