COMP: The install path for DoubleConversion lib was wrong.

As reported by Vladimir Chalupecky, the install path for
double-conversion libraries was wrong. I fixed and tested
that, and used the ITK Module-generated path variables
instead of ad hoc variables local to the DoubleConversion
Module.

Change-Id: Ic67cadf23277aff5254b3e878f1bea46a9e0e6da
Merge topic 'CleanUp_DoubleConversion_Install_Paths'

f228cd4 COMP: The install path for DoubleConversion lib was wrong.