Enable sourcekitd to be built by default on Linux
Enable sourcekitd to be built by default on Linux

The Linux build has a dependency on the libdispatch library,
which is needed by the various native libraries for sourcekitd.

On macOS, the dependency for libdispatch is satisfied directly through
the base OS, but on Linux no such dependency exists.

Modify this so that if the SourceKit library is built, and the
libdispatch library is already present, then we shell out to make
the libdispatch binary project when the SourceKit is built.

Issue: SR-1676

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