macro comment

Author: rcsanchez97

src/libbson/src/bson/bson-atomic.h

@@ -54,6 +54,11 @@ enum bson_memory_order {
 #undef BSON_USE_LEGACY_GCC_ATOMICS
 #endif
 
+/* Not all GCC-like compilers support the current __atomic built-ins.  Older
+ * GCC (pre-4.9) used different built-ins named with the __sync prefix.  When
+ * compiling with such older GCC versions, it is necessary to use the applicable
+ * functions, which requires redefining BSON_IF_GNU_LIKE and defining the
+ * additional BSON_IF_GNU_LEGACY_ATOMICS macro here. */
 #ifdef BSON_USE_LEGACY_GCC_ATOMICS
 #undef BSON_IF_GNU_LIKE
 #define BSON_IF_GNU_LIKE(...)