Check for the definition of `_MSC_VER`, rather than its value.
Merge pull request #153 from pqu/_msc_ver-def

Check for the definition of `_MSC_VER`, rather than its value.
Merge pull request #152 from pqu/cmake-fix

CMake: Fix #149.
CMake: Add support for CAPNP_LITE version of kj to support MSVC builds.
CMake: Update CMakeLists with MSVC compiler flags and paths.
CMake: Remove redundant MSVC test.

CMake automatically patches the switch character.
MSVC: Qualify `std::initializer_list` to fix name lookup when compiling 'iostream-test.c++'.
CMake: Fix paths to gtest libraries for the Xcode generator.
CMake: Reduce MSVC warning level