cmake: define variable for special-casing in Vc code

Vc cmake code needs to do some extra/different calls when inside ROOT.
In order to easily sync with upstream the Vc_INSIDE_ROOT variable is
used.

Signed-off-by: Matthias Kretz <kretz@kde.org>
Merge from Matthias github branch latest change for Vc 0.7
“update to latest Vc 0.7 branch”
Add again suppression of warning for unused function in Vc for latest clang version