Fix build with mingw 4.8.2 and qt 5.3.2, bug #14735

With mingw 4.8.2 that comes with qt 4.3.2, there is a pthread implementation,
this fix allows VTK to be built with this setup. The patch basically
turns off windows threads if pthreads are found.

Change-Id: Iaeefc6b317d50f6cdfbef7335bcd4b93a98df059
Merge topic 'fix-mingw482qt532-build' into master

14cfa485 Fix build with mingw 4.8.2 and qt 5.3.2, bug #14735