Fix YAML_DECLARE macro for MINGW32 architecture.

MinGW declares WIN32. Since `YAML_DECLARE_STATIC` isn't declared
before yaml.h is included, `YAML_DECLARE` will expand to

    define  YAML_DECLARE(type)  __declspec(dllexport) type

which doesn't work with MinGW, instead you would end up with
undefined references to `_impl__yaml_*`. Checking whether you're
currently in a MINGW32 setting fixes that behaviour.