msvc/cmake warning level

[Peter Kümmel]

The default warning level of the msvc cmake builds is now /W4.

The cmake option
    -DDISABLEWALL=1
switches to /W3,
    -DWALL=1
re enables /W4.

To disable a specific warning add it to MSVC_W_DISABLE of
cmake/CMakeLists.txt. To make the warning an error add it
to MSVC_W_ERROR of the same file.

Peter