Add support for GCC below 4.1.0.

src/civetweb.c

@@ -1253,7 +1253,7 @@ mg_atomic_inc(volatile int *addr)
 	 * (volatile unsigned int *) or (volatile LONG *),
 	 * so whatever you use, the other SDK is likely to raise a warning. */
 	ret = InterlockedIncrement((volatile long *)addr);
-#elif defined(__GNUC__)
+#elif defined(__GNUC__) && (__GNUC__ > 5 || (__GNUC__ == 4 && __GNUC_MINOR__ > 0))
 	ret = __sync_add_and_fetch(addr, 1);
 #else
 	ret = (++(*addr));
@@ -1270,7 +1270,7 @@ mg_atomic_dec(volatile int *addr)
 	 * (volatile unsigned int *) or (volatile LONG *),
 	 * so whatever you use, the other SDK is likely to raise a warning. */
 	ret = InterlockedDecrement((volatile long *)addr);
-#elif defined(__GNUC__)
+#elif defined(__GNUC__) && (__GNUC__ > 5 || (__GNUC__ == 4 && __GNUC_MINOR__ > 0))
 	ret = __sync_sub_and_fetch(addr, 1);
 #else
 	ret = (--(*addr));