windows: Set debug options if appropriate. (mozilla#798)

* windows: Set debug options if appropriate.

If building with debug, also build standard libraries with debug. This
ensures that any libraries linking against cubeb won't have mismatched
symbol definitions.

* Properly allow dynamic linking.

* Set global CMAKE_MSVC_RUNTIME_LIBRARY.

This variable will apply to all targets, rather than needing to apply a
per-target setting.

* remove unnecessary whitespace

* Simplify variable expression

CMakeLists.txt

@@ -1,7 +1,8 @@
 # TODO
 # - backend selection via command line, rather than simply detecting headers.
 
-cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.15 FATAL_ERROR)
+cmake_policy(SET CMP0091 NEW)
 project(cubeb
   VERSION 0.0.0)
 
@@ -12,6 +13,8 @@ option(BUILD_TOOLS "Build tools" ON)
 option(BUNDLE_SPEEX "Bundle the speex library" OFF)
 option(LAZY_LOAD_LIBS "Lazily load shared libraries" ON)
 option(USE_SANITIZERS "Use sanitizers" ON)
+# Set debugging for runtime libraries if requested.
+set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
 
 if(NOT CMAKE_BUILD_TYPE)
   set(CMAKE_BUILD_TYPE "RelWithDebInfo" CACHE STRING