Move debug symbols compile flag behind definition

Currently CFLAGS includes `-g` by default. LDFLAGS does not include it at all.

It would be better to do this in Makefile:

```
ifeq (${DEBUG},1)
CFLAGS += -g
LDFLAGS += -g
endif
```


I'll track this issue and make a PR when my load is a bit less
cc @rajames 