[SourceKit] don't build libdispatch shared library twice, other than on Mac/Windows

Author: finagolfin

CMakeLists.txt

@@ -941,7 +941,8 @@ if (LLVM_ENABLE_DOXYGEN)
   message(STATUS "Doxygen: enabled")
 endif()
 
-if(SWIFT_BUILD_SYNTAXPARSERLIB OR SWIFT_BUILD_SOURCEKIT)
+if((SWIFT_BUILD_SYNTAXPARSERLIB OR SWIFT_BUILD_SOURCEKIT) AND
+    ${SWIFT_HOST_VARIANT_SDK} MATCHES "OSX|WINDOWS")
   if(NOT CMAKE_SYSTEM_NAME STREQUAL Darwin)
     if(CMAKE_C_COMPILER_ID STREQUAL Clang AND
        CMAKE_C_COMPILER_VERSION VERSION_GREATER 3.8