Fix build against old systems

Signed-off-by: falkTX <falktx@falktx.com>

distrho/src/DistrhoPluginAU.cpp

@@ -44,6 +44,14 @@ START_NAMESPACE_DISTRHO
 
 // --------------------------------------------------------------------------------------------------------------------
 
+#ifndef __MAC_12_3
+enum {
+    kAudioUnitProperty_MIDIOutputBufferSizeHint = 66,
+};
+#endif
+
+// --------------------------------------------------------------------------------------------------------------------
+
 static const char* AudioUnitPropertyID2Str(const AudioUnitPropertyID prop) noexcept
 {
     switch (prop)

distrho/src/DistrhoPluginChecks.h

@@ -45,6 +45,8 @@
 // --------------------------------------------------------------------------------------------------------------------
 // Check that symbol macros are well defined
 
+#ifdef DISTRHO_PROPER_CPP11_SUPPORT
+
 #ifdef DISTRHO_PLUGIN_AU_TYPE
 static_assert(sizeof(STRINGIFY(DISTRHO_PLUGIN_AU_TYPE)) == 5, "The macro DISTRHO_PLUGIN_AU_TYPE has incorrect length");
 #endif
@@ -57,6 +59,8 @@ static_assert(sizeof(STRINGIFY(DISTRHO_PLUGIN_BRAND_ID)) == 5, "The macro DISTRH
 static_assert(sizeof(STRINGIFY(DISTRHO_PLUGIN_UNIQUE_ID)) == 5, "The macro DISTRHO_PLUGIN_UNIQUE_ID has incorrect length");
 #endif
 
+#endif
+
 // --------------------------------------------------------------------------------------------------------------------
 // Define optional macros if not done yet