build: allow building in Debug mode on Windows

With this tweak, we should be able to build the package in debug mode
even on Windows.  We always use the release mode DLL form of the C/C++
runtime on Windows and can build the Swift code in Debug or Release mode
optimizations.

CMakeLists.txt

@@ -1,10 +1,21 @@
+
+if(POLICY CMP0091)
+  cmake_policy(SET CMP0091 NEW)
+endif()
+
 cmake_minimum_required(VERSION 3.19)
 
 project(SwiftTSC LANGUAGES C Swift)
 
 set(CMAKE_Swift_LANGUAGE_VERSION 5)
 set(CMAKE_Swift_MODULE_DIRECTORY ${CMAKE_BINARY_DIR}/swift)
+# TODO: lift this restriction once we have a split Swift runtime build which can
+# be built with a debug C runtime.
+set(CMAKE_Swift_COMPILE_OPTIONS_MSVC_RUNTIME_LIBRARY MultiThreadedDLL)
 
+# TODO: lift this restriction once we have a split Swift runtime build which can
+# be built with a debug C runtime.
+set(CMAKE_MSVC_RUNTIME_LIBRARY MultiThreadedDLL)
 set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)