Include SwiftSupport when needed

Support for Swift is only needed when we have Swift enabled. We don't
need to include the module when it isn't available. With the change to
query the Swift compiler for the module triple, we need the Swift
language to be enabled to set the swift module triple.

Since the include was pulled from the main CMakeLists file, it's prudent
to set the include guard to prevent accidentally loading the module
multiple times.

Author: etcwilde

CMakeLists.txt

@@ -117,7 +117,6 @@ include(DispatchAppleOptions)
 include(DispatchSanitization)
 include(DispatchCompilerWarnings)
 include(DTrace)
-include(SwiftSupport)
 
 # NOTE(abdulras) this is the CMake supported way to control whether we generate
 # shared or static libraries.  This impacts the behaviour of `add_library` in

cmake/modules/SwiftSupport.cmake

@@ -1,3 +1,5 @@
+include_guard()
+
 if(NOT dispatch_MODULE_TRIPLE)
   set(module_triple_command "${CMAKE_Swift_COMPILER}" -print-target-info)
   if(CMAKE_Swift_COMPILER_TARGET)

src/swift/CMakeLists.txt

@@ -1,3 +1,5 @@
+include(SwiftSupport)
+
 if(HAVE_OBJC)
   add_library(DispatchStubs STATIC
     DispatchStubs.m)