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

    __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.

This behaviour was introduces by the libyaml update, which removed `define YAML_DECLARE_STATIC` (#583abc2f77fa9fb4dfb3c0e38faac08181851c42).