[polly][CMake] Replace `elseif ()` with `else ()`

The no-argument elseif resulted in the warning below:

CMake Warning (dev) at polly/lib/External/CMakeLists.txt:30 (elseif):
  ELSEIF called with no arguments, it will be skipped.

GitOrigin-RevId: 5109361d4aeeb0227e86a7f016eb773d28dd9957