d3d11va: Check WINAPI_FAMILY instead of HAVE_LOADLIBRARY

d3d11va: Check WINAPI_FAMILY instead of HAVE_LOADLIBRARY

If using the winstore compat library, a fallback LoadLibrary
function does exist, that only calls LoadPackagedLibrary though
(which doesn't work for dynamically loading d3d11 DLLs).

Therefore explicitly check the targeted API family instead.

Make this check a reusable HAVE_* component which other parts
of the libraries can check when necessary as well.

Signed-off-by: Martin Storsjö <martin@martin.st>
Merged from Libav commit 4d330da006fe48178.