Add link to pthreads for CosmoTools VTKExtensions

This commit adds linking to the pthreads library when building
the CosmoTools VTKExtensions, which is needed by the GenericIO
library v1.2 and above.

Change-Id: Ib1b9eace989fc98b6fbd7eecda72f3c1627b2297
Merge topic 'link-pthreads-for-genericio' into next

35e8b4aa Add link to pthreads for CosmoTools VTKExtensions