Pass -Djava.security.manager=allow from *nix launcher script for Java 18

src/script/ant

@@ -368,6 +368,12 @@ else
     ant_sys_opts="-Djikes.class.path=\"$JIKESPATH\""
   fi
 fi
+# Run "java -XshowSettings:properties" and check the output for "java.specification.version" value
+JAVA_SPEC_VERSION=`"$JAVACMD" -XshowSettings:properties 2>&1 | grep "java.specification.version = " | tr -d '[:space:]'`
+if [ "$JAVA_SPEC_VERSION" == "java.specification.version=18" ]; then
+  # set security manager property to allow calls to System.setSecurityManager() at runtime
+  ANT_OPTS="$ANT_OPTS -Djava.security.manager=allow"
+fi
 ant_exec_command="exec \"\$JAVACMD\" $ANT_OPTS -classpath \"\$LOCALCLASSPATH\" -Dant.home=\"\$ANT_HOME\" -Dant.library.dir=\"\$ANT_LIB\" $ant_sys_opts org.apache.tools.ant.launch.Launcher $ANT_ARGS -cp \"\$CLASSPATH\""
 if $ant_exec_debug; then
   # using printf to avoid echo line continuation and escape interpretation confusion