Improve FILE(DOWNLOAD ...):

- Add percent complete progress output to the FILE DOWNLOAD
  command. This progress output is off by default to
  preserve existing behavior. To turn it on, pass
  SHOW_PROGRESS as an argument.

- Add EXPECTED_MD5 argument. Verify that the downloaded
  file has the expected md5 sum after download is complete.

- Add documentation for SHOW_PROGRESS and EXPECTED_MD5.

  When the destination file exists already and has the
  expected md5 sum, then do not bother re-downloading
  the file. ("Short circuit" return.)

  Also, add a test that checks for the status output
  indicating that the short circuit behavior is actually
  occurring. Use a binary file for the test so that the
  md5 sum is guaranteed to be the same on all platforms
  regardless of "shifting text file line ending" issues.

Improve ExternalProject:

- Add argument URL_MD5.

- Add verify step that compares md5 sum of .tar.gz file
  before extracting it.

- Add md5 check to download step, too, to prevent
  unnecessary downloads.

- Emit a warning message when a file is not verified.
  Indicate that the file may be corrupt or that no
  checksum was specified.