And by "valid" i mean if it returns the double-conversion::double-conversion
target that we are going to use later. Fedora rawhide has a
doubleconversion cmake config file but it doesn't define any target
Change-Id: Ib0360bc9fdf593a14510c9a2d59b47b32bc4b356
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>