Fixes for C-level build of libdispatch on Linux#263

dgrove-oss wants to merge 1 commit intoswiftlang:masterswiftlang/swift-corelibs-libdispatch:masterfrom

dgrove-oss:dispatch-cmark-fixes-reduxdgrove-oss/swift-corelibs-libdispatch:dispatch-cmark-fixes-reduxCopy head branch name to clipboard