[SourceKit] Register optional sources in CMake

LLVM's CMake modules include a function `llvm_process_sources()`,
which (among other things) verifies that all source files in a
directory are either included in the list of source files to process,
or are included in a list `LLVM_OPTIONAL_SOURCES`.

SourceKit's CMake functions make use of this LLVM function, but do
not register any files as "optional". When attempting to configure
CMake to include SourceKit on a Linux host machine, source files
that are only included on Darwin host machines cause this function
to raise an error.

Mark Darwin-only SourceKit files as "optional" to avoid the error.

Author: modocache

tools/SourceKit/lib/Support/CMakeLists.txt

@@ -7,10 +7,11 @@ set(SourceKitSupport_sources
   UIDRegistry.cpp
 )
 
+set(SourceKitSupport_Darwin_sources
+  Concurrency-Mac.cpp)
+set(LLVM_OPTIONAL_SOURCES ${SourceKitSupport_Darwin_sources})
 if(APPLE)
-  list(APPEND SourceKitSupport_sources
-    Concurrency-Mac.cpp
-  )
+  list(APPEND SourceKitSupport_sources ${SourceKitSupport_Darwin_sources})
 endif()
 
 add_sourcekit_library(SourceKitSupport

tools/SourceKit/tools/sourcekitd/lib/API/CMakeLists.txt

@@ -7,10 +7,11 @@ set(sourcekitdAPI_sources
   TokenAnnotationsArray.cpp
 )
 
+set(sourcekitdAPI_Darwin_sources
+  sourcekitdAPI-XPC.cpp)
+set(LLVM_OPTIONAL_SOURCES ${sourcekitdAPI_Darwin_sources})
 if(APPLE)
-  list(APPEND sourcekitdAPI_sources
-    sourcekitdAPI-XPC.cpp
-  )
+  list(APPEND sourcekitdAPI_sources ${sourcekitdAPI_Darwin_sources})
 endif()
 
 add_sourcekit_library(sourcekitdAPI