fix SHLD spec for Windows

spec/build_tests_spec.rb

@@ -1190,9 +1190,13 @@ EOF
       result = run_test(rsconsfile: "shared_library_set_shld.rb")
       expect(result.stderr).to eq ""
       slines = lines(result.stdout)
+      if RUBY_PLATFORM =~ /mingw/
+        expect(slines).to include("SHLD mine.dll")
+      else
         expect(slines).to include("SHLD libmine.so")
       end
     end
+  end
 
   context "multi-threading" do
     it "waits for subcommands in threads for builders that support threaded commands" do