ENH: Missing download location in LibSVM external project

SuperBuild/CMake/External_libsvm.cmake

@@ -18,6 +18,7 @@ else()
     URL "http://www.csie.ntu.edu.tw/~cjlin/libsvm/libsvm-3.20.tar.gz"
     URL_MD5 5f088e5f89da1c65b642300c9c5ea772
     BINARY_DIR ${LIBSVM_SB_BUILD_DIR}
+    DOWNLOAD_DIR ${DOWNLOAD_LOCATION}
     INSTALL_DIR ${SB_INSTALL_PREFIX}
     CMAKE_CACHE_ARGS
       -DCMAKE_INSTALL_PREFIX:STRING=${SB_INSTALL_PREFIX}