Incorporate vdt

add the vdt mathematical library to root.
https://svnweb.cern.ch/trac/vdt
The library offers extremely optimised, vectorisable and approximate
implementation of the most commonly used functions in HEP.

Points relevant to the integration:
1) Add a stamp file in the lib directory to transparently use the
Makefile infrastructure
2) Add a comprehensive test leveraging the CMake infrastructure
3) The build of vdt as library ready to preload will be delegated
to a script
4) The array signatures have been removed for the moment in favour
of a header only solution