add a cmake option MSVC_RUNTIME_LIBRARY

[DuKeM-CSGO]

CMakelists.txt Line 286:

if(BUILD_SHARED_LIBS)
  set_target_properties(snappy PROPERTIES WINDOWS_EXPORT_ALL_SYMBOLS ON)
endif(BUILD_SHARED_LIBS)

May I ask for an additional option MSVC_RUNTIME_LIBRARY here? When building static libs CMAKE_CXX_FLAGS should includes /MTd, instead of CMake's default value /MDd.