Gpufit and Cpufit libraries export too many symbols on Linux about gpufit

[jkfindeisen]

For a more detailed description of the problem see for example: https://stackoverflow.com/questions/6538501/linking-two-shared-libraries-with-some-of-the-same-symbols

[superchromix]

The solution suggested on the stack overflow page: "Mark symbols with an appropriate visibility." would appear to be a good suggestion?

The .def files were used to avoid some earlier bugs we had with the binaries. They should stay for the time being.

[jkfindeisen]

9481b93 limits the export of symbols on gcc. The performance comparison application does not crash anymore.

However, all symbols in the Gpufit library from the CUDA code are still exported. This is because FindCUDA in CMake has a problem. See https://gitlab.kitware.com/cmake/cmake/issues/17533.

One solution would be to use CUDA first class support directly in CMake (not via find_package) from CMake 3.8 on but this didn't work right out of the box. So I will do this later. See https://cmake.org/cmake/help/v3.8/release/3.8.html#cuda