libretro: Fix GB games that uses serial (WIP)

Allow serial emulation without having the need to run link-related
communication stuff (NO_LINK define). Some games need this e.g. RC Pro-AM
racing. Dummy funcs/vars are added as placeholders.

Currently work-in-progress and might need to disable some more NO_LINK
sections.

src/libretro/Makefile

@@ -1,6 +1,7 @@
 TILED_RENDERING=0
 STATIC_LINKING=0
 FRONTEND_SUPPORTS_RGB565=1
+NO_LINK=0
 
 SPACE :=
 SPACE := $(SPACE) $(SPACE)
@@ -259,12 +260,16 @@ include Makefile.common
 
 OBJS := $(SOURCES_CXX:.cpp=.o)
 
-VBA_DEFINES += -D__LIBRETRO__ -DFINAL_VERSION -DC_CORE -DNO_LINK -DNO_DEBUGGER
+VBA_DEFINES += -D__LIBRETRO__ -DFINAL_VERSION -DC_CORE -DNO_DEBUGGER
 
 ifeq ($(FRONTEND_SUPPORTS_RGB565),1)
 VBA_DEFINES += -DFRONTEND_SUPPORTS_RGB565
 endif
 
+ifeq ($(NO_LINK),1)
+VBA_DEFINES += -DNO_LINK
+endif
+
 ifneq ($(SANITIZER),)
    CFLAGS += -fsanitize=$(SANITIZER)
    CXXFLAGS += -fsanitize=$(SANITIZER)

src/libretro/libretro.cpp

@@ -80,6 +80,49 @@ int emulating = 0;
 void (*dbgOutput)(const char* s, uint32_t addr);
 void (*dbgSignal)(int sig, int number);
 
+// Dummy vars/funcs for serial io emulation without LINK communication related stuff
+#ifndef NO_LINK
+#include "../gba/GBALink.h"
+uint8_t gbSIO_SC;
+bool LinkIsWaiting;
+bool LinkFirstTime;
+bool EmuReseted;
+int winGbPrinterEnabled;
+bool gba_joybus_active = false;
+
+LinkMode GetLinkMode()
+{
+    return LINK_DISCONNECTED;
+}
+
+void StartGPLink(uint16_t value)
+{
+}
+
+void LinkUpdate(int ticks)
+{
+}
+
+void StartLink(uint16_t siocnt)
+{
+}
+
+void CheckLinkConnection()
+{
+}
+
+void gbInitLink()
+{
+   LinkIsWaiting = false;
+   LinkFirstTime = true;
+}
+
+uint16_t gbLinkUpdate(uint8_t b, int gbSerialOn) //used on external clock
+{
+   return (b << 8);
+}
+#endif
+
 #define GS555(x) (x | (x << 5) | (x << 10))
 uint16_t systemGbPalette[24] = {
     GS555(0x1f), GS555(0x15), GS555(0x0c), 0,