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>

math/vc/CMakeLists.txt

+set(Vc_INSIDE_ROOT TRUE)
+
 include(cmake/VcMacros.cmake)
 include(cmake/AddTargetProperty.cmake)
 vc_determine_compiler()