Containers: fix LinkedList compilation errors on MSVC.

This issue appears only in Magnum::SceneGraph::Object, when the class
is derived from more than one linked list.
Utility: fix MSVC warning about out-of-range value.
Explicitly convert streamoff to size_t.

Fixes a few warnings on MSVC.
Utility: use `do while(false)` instead of `do while(0)`.

Looks better to me, but the useless warning on MSVC still persists.
Utility: don't return after std::abort().

Fixes "unreachable code" warnings on MSVC.
UseCorrade.cmake: some people might want to try C++11 on GCC -3.7.
UseCorrade.cmake: notify about support and compatibility mode on MSVC.

The actual support is done in compatibility branch.
UseCorrade.cmake: MSVC-specific compiler flags.