WL#5161: Implement Mats' suggestion of moving OS specific CMake code to OS specific files, instead of 
polluting code with IF(CMAKE_SYSTEM_NAME MATCHES...), first on Windows.

cmake/libutils.cmake:
  Fix the case in MERGE_LIBRARIES, where  there is no dependency on OS libraries.
cmake/os/Windows.cmake:
  Move windows specific code to cmake/os/Windows.cmake
configure.cmake:
  Move some Windows code to cmake/os/Windows.cmake