Vc: update to Vc 0.7.4

Summary of the changes in Vc 0.7.4:
* fixed several compile errors / warnings with newer or old C++
  compilers
* support clean compilation with more -W flags
* fixed compilation when compiling without optimization
* added operator-- to Vector<T>
* Copying Memory now uses SIMD move instructions
* Vc::Allocator<T> now uses a minimum alignment of the SIMD types of
  the chosen Vc implementation. Thus making it useable for containers of
  builtin types.

Signed-off-by: Matthias Kretz <kretz@kde.org>