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

Source/cmGeneratorTarget.cxx

@@ -1999,8 +1999,13 @@ void cmGeneratorTarget::ComputeModuleDefinitionInfo(
   info.WindowsExportAllSymbols =
     this->Makefile->IsOn("CMAKE_SUPPORT_WINDOWS_EXPORT_ALL_SYMBOLS") &&
     this->GetPropertyAsBool("WINDOWS_EXPORT_ALL_SYMBOLS");
+#if defined(_WIN32) && defined(CMAKE_BUILD_WITH_CMAKE)
   info.DefFileGenerated =
     info.WindowsExportAllSymbols || info.Sources.size() > 1;
+#else
+  // Our __create_def helper is only available on Windows.
+  info.DefFileGenerated = false;
+#endif
   if (info.DefFileGenerated) {
     info.DefFile = this->ObjectDirectory /* has slash */ + "exports.def";
   } else if (!info.Sources.empty()) {