Do not generate rules for .def generation where not supported

Our `cmake -E __create_def` helper used for `WINDOWS_EXPORT_ALL_SYMBOLS`
and merging of multiple `.def` files is available only with CMake hosted
on Windows.  However, we may generate use of it on other platforms since
commit v3.9.0-rc1~405^2 (Support WINDOWS_EXPORT_ALL_SYMBOLS with `.def`
files, 2017-03-13) when multiple `.def` files are listed.  This results
in a build error because the tool doesn't exist.

Fix our logic to avoid using the tool on non-Windows platforms.  Instead
silently ignore all but the first `.def` source as we did before.

Issue: #17773

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