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.

[Bernardo-Gabriel]

Are you running DUNE on OpenBSD? What system are you using?
At this moment, we are only actively maintaining builds using g++ on Linux-based OSs. I don't see any major issue regarding compatibility with OpenBSD, but be aware that we are not currently maintaining it.

Regarding the compilation error you mentioned.
Maybe it would be better to add /usr/local/ as an environment variable:
export CMAKE_PREFIX_PATH=/usr/local
Or force CMake to look at your /usr/local/:
cmake .. -DCMAKE_PREFIX_PATH=/usr/local