Trouble linking static library with Clang on Windows using dynamic runtime.

[hendrik-skubch]

lld-link: error: /failifmismatch: mismatch detected for 'RuntimeLibrary':

something.lib(something.cpp.obj) has value MDd_DynamicDebug
yaml-cppd.lib(convert.cpp.obj) has value MTd_StaticDebug

#741 seems to make it impossible to set YAML_MSVC_SHARED_RT if MSVC evaluates to false.