Allow usage of the new Condition Variable code with Critical Sections

Vista and later provide the SleepConditionVariableCS() function for this.

Since SDL_syscond_srw.c doesn't require SRW locks anymore, rename it to
SDL_syscond_cv.c which better reflects the implementation of condition
variables rather than the implementation of mutexes.

Fixes #4051.

CMakeLists.txt

@@ -1596,7 +1596,7 @@ elseif(WINDOWS)
     set(SDL_THREAD_WINDOWS 1)
     set(SOURCE_FILES ${SOURCE_FILES}
       ${SDL2_SOURCE_DIR}/src/thread/generic/SDL_syscond.c
-      ${SDL2_SOURCE_DIR}/src/thread/windows/SDL_syscond_srw.c
+      ${SDL2_SOURCE_DIR}/src/thread/windows/SDL_syscond_cv.c
       ${SDL2_SOURCE_DIR}/src/thread/windows/SDL_sysmutex.c
       ${SDL2_SOURCE_DIR}/src/thread/windows/SDL_syssem.c
       ${SDL2_SOURCE_DIR}/src/thread/windows/SDL_systhread.c