Fix compilation with DEBUG flag

- Use DEBUG flag similar to other libretro cores (DEBUG=0 disables,
  DEBUG=1 enables the debug build)
- Optimize with -O2 for debugging because the core won't work otherwise
- Always strip sections to prevent undefined symbol errors

Makefile

@@ -1,16 +1,19 @@
 TARGET ?= PicoDrive
+DEBUG = 0
 CFLAGS += -Wall 
 CFLAGS += -I. -DINLINE=inline
-ifndef DEBUG
+ifeq ($(DEBUG),0)
 ifeq ($(platform), vita)
 CFLAGS += -O3 -DNDEBUG
 else
 CFLAGS += -O2 -DNDEBUG -ffunction-sections
 endif
+else
+CFLAGS += -g -O2
+endif
 ifneq ($(APPLE),1)
 LDFLAGS += -Wl,--gc-sections
 endif
-endif
 #CFLAGS += -DEVT_LOG
 #CFLAGS += -DDRC_CMP
 #cpu_cmp = 1