cmake: add qnx support

Co-authored-by: elahav <elahav@users.noreply.github.com>

cmake/sdlplatform.cmake

@@ -28,6 +28,8 @@ macro(SDL_DetectCMakePlatform)
       set(SDL_CMAKE_PLATFORM AIX)
     elseif(CMAKE_SYSTEM_NAME MATCHES "Minix.*")
       set(SDL_CMAKE_PLATFORM MINIX)
+    elseif(CMAKE_SYSTEM_NAME MATCHES "QNX")
+      set(SDL_CMAKE_PLATFORM QNX)
     endif()
   elseif(APPLE)
     if(CMAKE_SYSTEM_NAME MATCHES ".*Darwin.*")