Add "wsl_clipboard" build tag for creating Linux binaries that use the Windows clipboard

src/backend/copyTERMUX.go

@@ -1,4 +1,4 @@
-//go:build termux
+//go:build termux && !windows && !darwin
 
 package backend
 

src/backend/copyUNIXGeneric.go

@@ -1,4 +1,4 @@
-//go:build !windows && !darwin && !termux
+//go:build !windows && !darwin && !termux && !wsl_clipboard
 
 package backend
 

src/backend/copyWIN.go

@@ -1,4 +1,4 @@
-//go:build windows
+//go:build windows || (!windows && !darwin && !termux && wsl_clipboard)
 
 package backend