Add support for OpenBSD to build with clang, #48
                
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basically link against -lc++abi when using clang.
I don't know if checking the value of CLANG_CC variable is the most optimal route,
but it was the best/most natural I could find. Open for suggestions otherwise.
I had it in a ugly way hardcoded in the GNUstep ports tree for years, since that doesn't support building with gcc ;)
Tested on amd64, with gcc 11 from packages, and linking against gcc libobjc, as well as using base clang, and building against libobjc2 from packages.