Prevent use of ant directly

With the logic for versioning added to build.py directly,
it is now somewhat confusing to mix uses of build.py and
ant. This commit permits only the use of build.py (or
specifically only runs of ant where -Dbuild.py=true).

build.py

@@ -89,6 +89,7 @@ def java_omero(args):
     command = [ find_java() ]
     p = os.path.join( os.path.curdir, "lib", "log4j-build.xml")
     command.append("-Dlog4j.configuration=%s" % p)
+    command.append("-Dbuild.py=true")
     command.extend( calculate_memory_args() )
     command.extend(["omero"])
     command.extend(choose_omero_version())

components/antlib/resources/global.xml

@@ -214,6 +214,19 @@
         <taskdef resource="net/sf/antcontrib/antlib.xml"
                  classpath="${lib.dir}/repository/ant-contrib-1.0b3.jar"/>
 
+        <!-- prevent direct use of ant -->
+        <if><not><isset property="build.py"/></not>
+            <then>
+                <fail> ERROR!
+
+========================================================
+Please use ./build.py which uses the ant version bundled
+with OMERO.
+========================================================
+                </fail>
+            </then>
+        </if>
+
         <!-- Define properties :
                 The user's environment (env) overrides all other
                 variables, then comes the user defined