Fix CMAKE_C_VISIBILITY_PRESET for cmake versions greater than 1.8

[hughbe]

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.

If this is not added, we get an error: error: unknown argument: '-fvisibility=hidden'

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

@jrose-apple could you help triage this :)

[hughbe]

Upstream pull: commonmark#162

[jrose-apple]

Oof. How did we get that wrong? Thanks, @hughbe. Let's see, do we have testing for CMark?

@swift-ci Please test

[jrose-apple]

I guess not. Let's wait a day to see if there's any feedback from upstream, and then I'll merge.

[jrose-apple]

Heh. Merged almost immediately upstream, so I'll merge here too. (We should probably consider pulling from upstream soon.)

[hughbe]

The upstream PR passed the CI but I'm not sure how similar this fork is to upstream. I'll see if I can pull upstream later tonight and send a PR for that

[Nader822]

H