Oops, forgot /reference:

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@278127 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/taskdefs/optional/dotnet/DotnetCompile.java

@@ -222,7 +222,7 @@ public abstract class DotnetCompile
             if (isWindows) {
                 return REFERENCE_OPTION + '\"' + references + '\"';
             } else {
-                return references;
+                return REFERENCE_OPTION + references;
             }
         } else {
             return null;