Fix compilation on OpenBSD

[thyssentishman]

I had to make this change to get DUNE to compile on OpenBSD. Perhaps you'd like to merge it.

On a related note, I had to run CMake as follows (OpenBSD libraries are installed under /usr/local/lib):

mkdir build && cd build && CXXFLAGS="-L/usr/local/lib" cmake ..

Otherwise, I would get the following error:

ld: error: unable to find library -lcurl

Not sure where I'd patch this in the CMake files.