Provisioning C libraries in Windows CI builds

[Per Nordlöw]

I just tried activating Linux and Windows CI for

https://code.dlang.org/packages/gmp-d

via

https://github.com/nordlow/gmp-d/.github/workflows/d.yml

Linux passes but the Windows builds all fail because `gmp.lib` is 
not provisioned in the Windows CI. This is new to me so I need 
some initial guidance as I'm suspecting there are multiple 
package managers or direct sources download methods available for 
getting gmp.lib provisioned locally so linking finds `gmp.lib`.

gmp is available via https://vcpkg.io/en/packages.html is that 
the preferred way?