dev-libs/libyaml: fix dll build on mingw-w64.

dev-libs/libyaml: fix dll build on mingw-w64.

includes the following patches:

https://github.com/yaml/libyaml/commit/119b7b6adfa72552d6dc7eb03f402ff5cf8f9fa6
https://github.com/yaml/libyaml/pull/85

Former is upstreamed, the latter is a pending PR. Without these changes a
shared library/dll cannot be created on mingw-w64. Latter patch is applied also
to the generated Makefile.in in order to prevent the need for eautoreconf.

Package-Manager: Portage-2.3.19, Repoman-2.3.6
Closes: https://github.com/gentoo/gentoo/pull/6752