My other PR revealed, that the /W4 parameter, we give to MSVC is ignored
because cmake set CMAKE_C_FLAGS already to /W3 and we overwrite it.
The command line tools gave me a D9025 warning for this on every file and
looking into the project properties page on MSVC 2008 tells, that it has
the warning level set to /W3.
However, the warnings introduced by /W4 are far to useless for having
them enabled. So just disable them.
