In a static library build with clang-cl on Windows, clang-cl warns that avif.lib is empty

[wantehchang]

In a static library build (-DBUILD_SHARED_LIBS=OFF) with clang-cl on Windows, clang-cl warns that avif.lib is empty.

See https://github.com/AOMediaCodec/libavif/actions/runs/9999096788/job/27639410608?pr=2298 for an example:

[62/161] Linking C static library avif.lib
ignoring unknown argument: -M

warning: no input files, not writing output file

         pass /llvmlibempty to write empty .lib file,

         pass /ignore:emptyoutput to suppress warning

I have not seen this warning in my local build. I don't know why.