Use a more robust method of obtaining the build timestamp on Windows. 'wmic os get...

Use a more robust method of obtaining the build timestamp on Windows.  'wmic os get LocalDateTime' will always return the timestamp in the format we want (YYYYMMDD), whereas date /t is sensitive to locale.  If wmic fails, then we fall back to using date /t, even though this means that the BUILD variable will end up in the incorrect format on some systems.

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/trunk@867 632fc199-4ca6-4c93-a231-07263d6284db