cmake: Avoid MSVC C4273 warnings in send/recv checks

We use `check_c_source_compiles` to check possible send/recv signatures
by reproducing the forward declarations from system headers.  On Windows
the `winsock2.h` header adds dll linkage settings to its forward
declaration.  If ours does not match the compiler warns:

    warning C4273: 'recv': inconsistent dll linkage

Add `WINSOCK_API_LINKAGE` to our test signatures when it is defined so
that our linkage is consistent with that from `winsock2.h`.

Fixes #4764
Closes #5232

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