Treat the QNX compiler as GCC for using atomics

CMakeLists.txt

@@ -201,7 +201,7 @@ else()
   set(SDL_ASSEMBLY_DEFAULT FALSE)
 endif()
 
-if(USE_GCC OR USE_CLANG OR USE_INTELCC)
+if(USE_GCC OR USE_CLANG OR USE_INTELCC OR USE_QCC)
   set(OPT_DEF_GCC_ATOMICS ON)
 endif()
 
@@ -545,7 +545,7 @@ if(NOT SDL_FOREGROUNDING_SIGNAL STREQUAL "OFF")
 endif()
 
 # Compiler option evaluation
-if(USE_GCC OR USE_CLANG OR USE_INTELCC)
+if(USE_GCC OR USE_CLANG OR USE_INTELCC OR USE_QCC)
   # Check for -Wall first, so later things can override pieces of it.
   # Note: clang-cl treats -Wall as -Weverything (which is very loud),
   #       /W3 as -Wall, and /W4 as -Wall -Wextra.  So: /W3 is enough.