Fix CMAKE_C_VISIBILITY_PRESET for cmake versions greater than 1.8 (e.g. 3.6.2) (#162)

This lets us build swift-cmark on Windows, using clang-cl

This was an oversight: CMAKE_C_VISIBILITY_PRESET is only available on CMAKE 1.8 or greater. However, the current check doesn't work for a version such as CMAKE 3.6.2, for example

```bash
mkdir "C:/Users/hughb/Documents/GitHub/my-swift/build/Ninja-DebugAssert/cmark-windows-amd64"
pushd "C:/Users/hughb/Documents/GitHub/my-swift/build/Ninja-DebugAssert/cmark-windows-amd64"
cmake -G Ninja -DCMAKE_C_COMPILER="C:/Program Files/LLVM/msbuild-bin/cl.exe" -DCMAKE_CXX_COMPILER="C:/Program Files/LLVM/msbuild-bin/cl.exe" -DCMAKE_CXX_FLAGS="-Xclang -std=c++14" "C:/Users/hughb/Documents/GitHub/my-swift/cmark"
popd
cmake --build "C:/Users/hughb/Documents/GitHub/my-swift/build/Ninja-DebugAssert/cmark-windows-amd64/" -- -j6 all
```

0 files changed, 0 insertions, 0 deletions