CMake: set MSVC warning level to 4

The MSVC warning level defaults to 3 in CMake. Change it to 4, which is
consistent with the Visual Studio and NMake builds. Disable level 4
warning C4127 for the library and additionally C4306 for the test
servers to get a clean CURL_WERROR build as that warning is raised in
some macros in older Visual Studio versions.

Ref: https://github.com/curl/curl/pull/1667#issuecomment-314082794
Closes https://github.com/curl/curl/pull/1711