Adds an option to select the CRT link mode ( static or dynamic ).
This is useful when linking libgit2 statically, as the setting must match the linking program's one.
Fix for a cmake bug when using MSVC + Win64 + static libraries
(see http://public.kitware.com/Bug/view.php?id=11240)
Merge pull request #1418 from QbProg/qb/cmake-msvc

MSVC cmake improvements