Move SWIFT_CONCURRENCY_USES_DISPATCH to StdlibOptions.cmake

It is only used in the stdlib build, so really has no business being set in the root `CMakeLists.txt`.

CMakeLists.txt

@@ -691,13 +691,6 @@ if(NOT EXISTS "${SWIFT_PATH_TO_SWIFT_SYNTAX_SOURCE}")
   message(SEND_ERROR "swift-syntax is required to build the Swift compiler. Please run update-checkout or specify SWIFT_PATH_TO_SWIFT_SYNTAX_SOURCE")
 endif()
 
-# Use dispatch as the system scheduler by default.
-# For convenience, we set this to false when concurrency is disabled.
-set(SWIFT_CONCURRENCY_USES_DISPATCH FALSE)
-if(SWIFT_ENABLE_EXPERIMENTAL_CONCURRENCY AND "${SWIFT_CONCURRENCY_GLOBAL_EXECUTOR}" STREQUAL "dispatch")
-  set(SWIFT_CONCURRENCY_USES_DISPATCH TRUE)
-endif()
-
 set(SWIFT_BUILD_HOST_DISPATCH FALSE)
 if(SWIFT_ENABLE_DISPATCH AND NOT CMAKE_SYSTEM_NAME STREQUAL Darwin)
   # Only build libdispatch for the host if the host tools are being built and
@@ -706,9 +699,9 @@ if(SWIFT_ENABLE_DISPATCH AND NOT CMAKE_SYSTEM_NAME STREQUAL Darwin)
     set(SWIFT_BUILD_HOST_DISPATCH TRUE)
   endif()
 
-  if(SWIFT_BUILD_HOST_DISPATCH OR SWIFT_CONCURRENCY_USES_DISPATCH)
+  if(SWIFT_BUILD_HOST_DISPATCH)
     if(NOT EXISTS "${SWIFT_PATH_TO_LIBDISPATCH_SOURCE}")
-      message(SEND_ERROR "SourceKit and concurrency require libdispatch on non-Darwin hosts.  Please specify SWIFT_PATH_TO_LIBDISPATCH_SOURCE")
+      message(SEND_ERROR "SourceKit requires libdispatch on non-Darwin hosts.  Please specify SWIFT_PATH_TO_LIBDISPATCH_SOURCE")
     endif()
   endif()
 endif()

stdlib/cmake/modules/StdlibOptions.cmake

@@ -239,3 +239,16 @@ set(SWIFT_RUNTIME_FIXED_BACKTRACER_PATH "" CACHE STRING
   "If set, provides a fixed path to the swift-backtrace binary.  This
    will disable dynamic determination of the path and will also disable
    the setting in SWIFT_BACKTRACE.")
+
+# Use dispatch as the system scheduler by default.
+# For convenience, we set this to false when concurrency is disabled.
+set(SWIFT_CONCURRENCY_USES_DISPATCH FALSE)
+if(SWIFT_ENABLE_EXPERIMENTAL_CONCURRENCY AND "${SWIFT_CONCURRENCY_GLOBAL_EXECUTOR}" STREQUAL "dispatch")
+  set(SWIFT_CONCURRENCY_USES_DISPATCH TRUE)
+endif()
+
+if(SWIFT_CONCURRENCY_USES_DISPATCH)
+  if(NOT EXISTS "${SWIFT_PATH_TO_LIBDISPATCH_SOURCE}")
+    message(SEND_ERROR "Concurrency require libdispatch on non-Darwin hosts.  Please specify SWIFT_PATH_TO_LIBDISPATCH_SOURCE")
+  endif()
+endif()