Mono 0.28 on MacOS X comes with a JIT but fails miserably, disable it

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@275420 13f79535-47bb-0310-9956-ffa450edef68

src/etc/testcases/taskdefs/optional/dotnet.xml

@@ -90,10 +90,15 @@
    <echo> dotnetapps.found=${dotnetapps.found}</echo>
 
    <condition property="mono.executable" value="mono">
-      <or>
-        <available file="mono"     filepath="${env.PATH}" />
-        <available file="mono.exe" filepath="${env.PATH}" />
-      </or>
+      <and>
+        <not>
+          <os family="mac"/>
+        </not>
+        <or>
+          <available file="mono"     filepath="${env.PATH}" />
+          <available file="mono.exe" filepath="${env.PATH}" />
+        </or>
+      </and>
    </condition>
    <property name="mono.executable" value="mint"/>
   </target>