The libyaml setup is currently configured improperly, not selecting
either shared nor static builds, resulting in libyaml being built
assuming that it is a user of libyaml, which is incorrect. Adjust the
build to build libyaml as a shared library.
This change is not exactly precise, over generalizing the application of
the `YAML_DECLARE_EXPORT` to avoid bumping the minimum version of SPM to
5.4, which is the first version to include support for
`.platform(.windows)`. This should not impact the other targets, as the
macro is not consulted for other targets.
This should allow us to enable testing on Windows again which was
disabled with #304.
Saleem Abdulrasool