build: various adjustments for the runtime libraries #874
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As per the discussion with @rokhinip, @mikeash, and @compnerd, always build libdispatch and libclosure in shared mode. The rationale for this is the following: If someone does a `-static-stdlib` build of Swift code as a plugin which is hosted in a C/C++ environment, you cannot have two dispatch implementations to run this. You need a single one, and because the host environment may be dynamically linked - you need to dynamically link libdispatch (the C portions). This prepares us to be able to build the rest of the dispatch code (swiftDispatch) both statically and dynamically.
mikeash approved these changes Sep 3, 2025
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Adjust the build in three specific manners:
BUILD_SHARED_LIBSonly permits us to globally control that behaviourAfter a side discussion between @rokhinip, @mikeash, @akmorrison, and myself (@compnerd), we decided that we should only support a dynamically linked libdispatch for the C portion of the runtime. While we have not yet decided on the direction for libclosure, in order to support the build of a static and dynamic libclosure, we pin the libdispatch build to a shared only build rather than just letting the build system control this via the
BUILD_SHARED_LIBSglobal flag.