rtapi: fix wrong STDC macro and warn when compiler is older than C11.

Author: BsAtHome

src/rtapi/rtapi_atomic.h

@@ -18,14 +18,16 @@
 
 #if defined(__GNUC__) && ((__GNUC__ << 8) | __GNUC_MINOR__) >= 0x409
 #define RTAPI_USE_STDATOMIC
-#elif defined(__STDC_VERSION__) && __STDC_VERSION > 201112L
+#elif defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
 #define RTAPI_USE_STDATOMIC
 #endif
 
 #ifdef RTAPI_USE_STDATOMIC
 #include <stdatomic.h>
 #else
 
+#warning "Old compiler has no C11 atomics. Please consider upgrading your compiler."
+
 enum memory_order {
     memory_order_relaxed,
     memory_order_consume,