Add QNX as a platform to the cmake build files

cmake/sdlchecks.cmake

@@ -807,6 +807,8 @@ macro(CheckPTHREAD)
     elseif(EMSCRIPTEN)
       set(PTHREAD_CFLAGS "-D_REENTRANT -pthread")
       set(PTHREAD_LDFLAGS "-pthread")
+    elseif(QNX)
+      # pthread support is baked in
     else()
       set(PTHREAD_CFLAGS "-D_REENTRANT")
       set(PTHREAD_LDFLAGS "-lpthread")

cmake/sdlplatform.cmake

@@ -54,6 +54,8 @@ macro(SDL_DetectCMakePlatform)
     set(SDL_CMAKE_PLATFORM Android)
   elseif(CMAKE_SYSTEM_NAME MATCHES "Emscripten.*")
     set(SDL_CMAKE_PLATFORM Emscripten)
+  elseif(CMAKE_SYSTEM_NAME MATCHES "QNX.*")
+    set(SDL_CMAKE_PLATFORM QNX)
   elseif(CMAKE_SYSTEM_NAME MATCHES "BeOS.*")
     message_error("BeOS support has been removed as of SDL 2.0.2.")
   endif()