[CMake] .def files since CMake 3.10 in Linux

[Patrick Boettcher]

Hi,

I (and someone else) stumbled upon a problem when using the Azure IOTHUB
SDK for C on a Linux platform using cmake 3.10+:

https://github.com/Azure/azure-iot-sdk-c/issues/505

The problem seems to be triggered by having two .def-files as
add_library()-source-files.

During the build cmake generates a command which does:

  cmake -E __create_def

This fails as unknown to cmake.

Is this a cmake-bug or is Azure using .def-files the wrong way?

A quick additional test show that the problem is coming from having
2 .def-files as sources. Having just one is OK.

--
Patrick.