Put AviWriter and its imports (and usages) behind #if AVI_SUPPORT

for now it's defined even on Linux, try removing it in /src/MainSlnCommon.props

src/BizHawk.Client.EmuHawk/AVOut/AviWriter.cs

@@ -1,4 +1,5 @@
-using System;
+#if AVI_SUPPORT
+using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
@@ -460,10 +461,12 @@ namespace BizHawk.Client.EmuHawk
 
 			public static void DeallocateAVICOMPRESSOPTIONS(ref AVIWriterImports.AVICOMPRESSOPTIONS opts)
 			{
+#endif
 #if false // test: increase stability by never freeing anything, ever
 				if (opts.lpParms != IntPtr.Zero) Win32Imports.HeapFree(Win32Imports.GetProcessHeap(), 0, opts.lpParms);
 				if (opts.lpFormat != IntPtr.Zero) Win32Imports.HeapFree(Win32Imports.GetProcessHeap(), 0, opts.lpFormat);
 #endif
+#if AVI_SUPPORT
 				opts.lpParms = IntPtr.Zero;
 				opts.lpFormat = IntPtr.Zero;
 			}
@@ -985,6 +988,7 @@ namespace BizHawk.Client.EmuHawk
 
 		public bool UsesVideo => true;
 
+#endif
 #if false // API has changed
 		private static void TestAVI()
 		{
@@ -1017,5 +1021,7 @@ namespace BizHawk.Client.EmuHawk
 			aw.CloseFile();
 		}
 #endif
+#if AVI_SUPPORT
 	}
 }
+#endif

src/BizHawk.Client.EmuHawk/AVOut/SynclessRecordingTools.cs

@@ -12,13 +12,18 @@ namespace BizHawk.Client.EmuHawk
 {
 	public partial class SynclessRecordingTools : Form, IDialogParent
 	{
-		private readonly List<FrameInfo> _mFrameInfos = new List<FrameInfo>();
 		private readonly Config _config;
+
 		private readonly IGameInfo _game;
 
-		private string _mSynclessConfigFile;
+#if AVI_SUPPORT
+		private readonly List<FrameInfo> _mFrameInfos = new List<FrameInfo>();
+
 		private string _mFramesDirectory;
 
+		private string _mSynclessConfigFile;
+#endif
+
 		public IDialogController DialogController { get; }
 
 		public SynclessRecordingTools(Config config, IGameInfo game, IDialogController dialogController)
@@ -29,6 +34,7 @@ namespace BizHawk.Client.EmuHawk
 			InitializeComponent();
 		}
 
+#if AVI_SUPPORT
 		public void Run()
 		{
 			var ofd = new OpenFileDialog
@@ -97,9 +103,11 @@ namespace BizHawk.Client.EmuHawk
 			public string WavPath { get; set; }
 			public string PngPath { get; set; }
 		}
+#endif
 
 		private void BtnExport_Click(object sender, EventArgs e)
 		{
+#if AVI_SUPPORT
 			if (_mFrameInfos.Count == 0)
 			{
 				return;
@@ -151,6 +159,7 @@ namespace BizHawk.Client.EmuHawk
 			}
 
 			avw.CloseFile();
+#endif
 		}
 	}
 }

src/BizHawk.Client.EmuHawk/MainForm.Designer.cs

@@ -869,7 +869,6 @@ namespace BizHawk.Client.EmuHawk
 			// SynclessRecordingMenuItem
 			// 
 			this.SynclessRecordingMenuItem.Text = "S&yncless Recording Tools";
-			this.SynclessRecordingMenuItem.Click += new System.EventHandler(this.SynclessRecordingMenuItem_Click);
 			// 
 			// ScreenshotSubMenu
 			// 

src/BizHawk.Client.EmuHawk/MainForm.Events.cs

@@ -532,11 +532,6 @@ namespace BizHawk.Client.EmuHawk
 			StopAv();
 		}
 
-		private void SynclessRecordingMenuItem_Click(object sender, EventArgs e)
-		{
-			new SynclessRecordingTools(Config, Game, this).Run();
-		}
-
 		private void CaptureOSDMenuItem_Click(object sender, EventArgs e)
 		{
 			bool c = ((ToolStripMenuItem)sender).Checked;

src/BizHawk.Client.EmuHawk/MainForm.cs

@@ -344,6 +344,11 @@ namespace BizHawk.Client.EmuHawk
 			ZXSpectrumExportSnapshotMenuItemMenuItem.Enabled = false;
 			ZXSpectrumExportSnapshotMenuItemMenuItem.Visible = false;
 #endif
+#if AVI_SUPPORT
+			SynclessRecordingMenuItem.Click += (_, _) => new SynclessRecordingTools(Config, Game, this).Run();
+#else
+			SynclessRecordingMenuItem.Enabled = false;
+#endif
 
 			Game = GameInfo.NullInstance;
 			_throttle = new Throttle();
@@ -3229,7 +3234,11 @@ namespace BizHawk.Client.EmuHawk
 
 			try
 			{
+#if AVI_SUPPORT
 				bool usingAvi = aw is AviWriter; // SO GROSS!
+#else
+				const bool usingAvi = false;
+#endif
 
 				if (_dumpaudiosync)
 				{

src/BizHawk.Common/Win32/AVIWriterImports.cs

@@ -1,5 +1,6 @@
 #nullable disable
 
+#if AVI_SUPPORT
 using System;
 using System.Runtime.InteropServices;
 
@@ -139,3 +140,4 @@ namespace BizHawk.Common
 		public static extern int AVIStreamWrite(IntPtr pavi, int lStart, int lSamples, IntPtr lpBuffer, int cbBuffer, int dwFlags, IntPtr plSampWritten, out int plBytesWritten);
 	}
 }
+#endif

src/MainSlnCommon.props

@@ -3,6 +3,7 @@
 	<PropertyGroup>
 		<AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
 		<CodeAnalysisRuleSet>$(ProjectDir)../../Common.ruleset</CodeAnalysisRuleSet>
+		<DefineConstants>$(DefineConstants);AVI_SUPPORT</DefineConstants>
 		<DocumentationFile>$(ProjectDir)bin/doc_comments.xml</DocumentationFile>
 		<MSBuildWarningsAsMessages>NU1702</MSBuildWarningsAsMessages>
 		<NoWarn>CS1573;CS1591</NoWarn>