One week ago (Jul 22) I added code to the CMakeLists.txt to clear
VTK_HAS_STD_ISFINITE, VTK_HAS_STD_ISNAN, and VTK_HAS_STD_ISINF
from the cache, in order to avoid dashboard build errors from
machines that had cached the incorrect values for these variables.
Now that all the dashboards have run, these variable can be persistent
in the cache like most other cache variables.

Change-Id: I38084b25da3328b4f2b833b5654653688b1eb2c5