From https://github.com/Ardour/ardour/pull/822/commits/c2ac82bfbacc583c49b306826dedec754633bbe7 Mon Sep 17 00:00:00 2001
From: Violet Purcell <vimproved@inventati.org>
Date: Thu, 21 Sep 2023 21:09:30 -0400
Subject: [PATCH] Backport VST3 libc++ fix on linux

LLVM libc++ does not have the ext/atomicity.h header. This fix is copied
from the upstream vst3_pluginterfaces repo.

Signed-off-by: Violet Purcell <vimproved@inventati.org>
--- a/libs/vst3/pluginterfaces/base/funknown.cpp
+++ b/libs/vst3/pluginterfaces/base/funknown.cpp
@@ -44,13 +44,23 @@
 #endif
 
 #if SMTG_OS_LINUX
+#if !defined (SMTG_USE_ATOMIC)
+#if defined (_LIBCPP_VERSION)
+#define SMTG_USE_ATOMIC 1
+#else
 #include <ext/atomicity.h>
+#endif
+#endif
 /* UUID */
 #include <string>
 #include <boost/uuid/uuid.hpp>
 #include <boost/uuid/uuid_generators.hpp>
 #endif
 
+#if defined (SMTG_USE_ATOMIC) && SMTG_USE_ATOMIC
+#include <atomic>
+#endif
+
 namespace Steinberg {
 
 //------------------------------------------------------------------------
@@ -79,7 +89,9 @@ namespace FUnknownPrivate {
 //------------------------------------------------------------------------
 int32 PLUGIN_API atomicAdd (int32& var, int32 d)
 {
-#if SMTG_OS_WINDOWS
+#if SMTG_USE_ATOMIC
+	return atomic_fetch_add (reinterpret_cast<std::atomic_int_least32_t*> (&var), d) +d;
+#elif SMTG_OS_WINDOWS
 	return InterlockedExchangeAdd ((volatile long int*)&var, d) + d;
 #elif SMTG_OS_MACOS
 	return OSAtomicAdd32Barrier (d, (int32_t*)&var);
-- 
2.42.0