Fix check to use a clean build

scripts/dashboard/common.cmake

@@ -357,9 +357,9 @@ if(COMMAND dashboard_hook_init)
 endif()
 
 if(dashboard_fresh)
-  if(EXISTS CTEST_BINARY_DIRECTORY)
+  if(EXISTS "${CTEST_BINARY_DIRECTORY}")
     message("Clearing build tree...")
-    ctest_empty_binary_directory(${CTEST_BINARY_DIRECTORY})
+    ctest_empty_binary_directory("${CTEST_BINARY_DIRECTORY}")
   else()
     file(MAKE_DIRECTORY "${CTEST_BINARY_DIRECTORY}")
   endif()