[yaml-cpp] warnings when using dynamic library

@Cheney-W is already working on this.

Since Nov 26, 2019.

When using yaml-cpp built as a dynamic library under visual studio there are warning issued C4251 (needs to have dll interface) and C4275 (non-dll interface used as base for dll-interface). This seems to stem from an action done in the portfile

When building yaml-cpp the portfile patches the file dll.h lines 58-64

file(READ ${CURRENT_PACKAGES_DIR}/include/yaml-cpp/dll.h DLL_H)
if(VCPKG_LIBRARY_LINKAGE STREQUAL "dynamic")
string(REPLACE "#ifdef YAML_CPP_DLL" "#if 1" DLL_H "${DLL_H}")
else()
string(REPLACE "#ifdef YAML_CPP_DLL" "#if 0" DLL_H "${DLL_H}")
endif()
file(WRITE ${CURRENT_PACKAGES_DIR}/include/yaml-cpp/dll.h "${DLL_H}")

This probably doesn't matter on static builds but for dynamic builds this cause the dll.h file to define

#define YAML_CPP_API __declspec(dllexport)

unconditionally, which in turn causes visual studio to issue a warning when using the yaml-cpp file. This block was introduced last year. I am assuming to fix another issue but there isn't really any explanation in the log, i'd just remove the block from the portfile, but it's hard to know what might get broken. Additionally it doesn't look like `YAML_CPP_DLL` gets defined in the portfile on dynamic builds. That should probably be passed to `vcpkg_configure_cmake`