diff --git a/Makefile b/Makefile
index 6252664bde..1c7eb46016 100644
--- a/Makefile
+++ b/Makefile
@@ -115,7 +115,7 @@ INCLUDE_SCANINC_ARGS := $(INCLUDE_DIRS:%=-I %)
 O_LEVEL ?= 2
 CPPFLAGS := $(INCLUDE_CPP_ARGS) -Wno-trigraphs -DMODERN=$(MODERN)
 ifeq ($(MODERN),0)
-  CPPFLAGS += -I tools/agbcc/include -I tools/agbcc -nostdinc -undef
+  CPPFLAGS += -I tools/agbcc/include -I tools/agbcc -nostdinc -undef -std=gnu89
   CC1 := tools/agbcc/bin/agbcc$(EXE)
   override CFLAGS += -mthumb-interwork -Wimplicit -Wparentheses -Werror -O$(O_LEVEL) -fhex-asm -g
   LIBPATH := -L ../../tools/agbcc/lib
diff --git a/include/gba/m4a_internal.h b/include/gba/m4a_internal.h
index a4182c8889..a90771048d 100644
--- a/include/gba/m4a_internal.h
+++ b/include/gba/m4a_internal.h
@@ -170,7 +170,7 @@ struct SoundChannel
 
 struct MusicPlayerInfo;
 
-#if !MODERN
+#if __STDC_VERSION__ < 202311L
 typedef void (*MPlayFunc)();
 #else
 typedef void (*MPlayFunc)(...);
diff --git a/include/librfu.h b/include/librfu.h
index 0bd4dc6c0d..8125d64043 100644
--- a/include/librfu.h
+++ b/include/librfu.h
@@ -316,7 +316,7 @@ struct STWIStatus
     u8 recoveryCount;
     u8 unk_16;
     u8 unk_17;
-#if !MODERN
+#if __STDC_VERSION__ < 202311L
     void (*callbackM)();
 #else
     void (*callbackM)(...);
diff --git a/src/librfu_intr.c b/src/librfu_intr.c
index 0426e6459c..5ae53ab4e3 100644
--- a/src/librfu_intr.c
+++ b/src/librfu_intr.c
@@ -7,7 +7,7 @@ static u16 handshake_wait(u16 slot);
 static void STWI_set_timer_in_RAM(u8 count);
 static void STWI_stop_timer_in_RAM(void);
 static void STWI_init_slave(void);
-#if !MODERN
+#if __STDC_VERSION__ < 202311L
 static void Callback_Dummy_M(int reqCommandId, int error, void (*callbackM)());
 #else
 static void Callback_Dummy_M(int reqCommandId, int error, void (*callbackM)(...));
@@ -139,7 +139,7 @@ static void sio32intr_clock_master(void)
         }
         gSTWIStatus->sending = 0;
         if (gSTWIStatus->callbackM != NULL)
-#if !MODERN
+#if __STDC_VERSION__ < 202311L
             Callback_Dummy_M(gSTWIStatus->reqActiveCommand, gSTWIStatus->error, gSTWIStatus->callbackM);
 #else
             Callback_Dummy_M(gSTWIStatus->reqActiveCommand, gSTWIStatus->error, (void (*)(...))gSTWIStatus->callbackM);
@@ -396,7 +396,7 @@ static void STWI_init_slave(void)
 }
 
 NAKED
-#if !MODERN
+#if __STDC_VERSION__ < 202311L
 static void Callback_Dummy_M(int reqCommandId, int error, void (*callbackM)())
 #else
 static void Callback_Dummy_M(int reqCommandId, int error, void (*callbackM)(...))
diff --git a/src/m4a.c b/src/m4a.c
index 1344f53080..2ad2261d08 100644
--- a/src/m4a.c
+++ b/src/m4a.c
@@ -283,7 +283,7 @@ void MPlayExtender(struct CgbChannel *cgbChans)
 
     soundInfo->ident++;
 
-#if !MODERN
+#if __STDC_VERSION__ < 202311L
     gMPlayJumpTable[8] = ply_memacc;
     gMPlayJumpTable[17] = ply_lfos;
     gMPlayJumpTable[19] = ply_mod;
@@ -332,7 +332,7 @@ void MusicPlayerJumpTableCopy(void)
 
 void ClearChain(void *x)
 {
-#if !MODERN
+#if __STDC_VERSION__ < 202311L
     void (*func)(void *) = *(&gMPlayJumpTable[34]);
 #else
     void (*func)(...) = *(&gMPlayJumpTable[34]);
@@ -342,7 +342,7 @@ void ClearChain(void *x)
 
 void Clear64byte(void *x)
 {
-#if !MODERN
+#if __STDC_VERSION__ < 202311L
     void (*func)(void *) = *(&gMPlayJumpTable[35]);
 #else
     void (*func)(...) = *(&gMPlayJumpTable[35]);