Improve modern res API available condition

`__RES` is usable to check release date of resolver library.
Such modern res API appeared in BIND-8.2.0 libbind with __RES == 19980901,
it was refrected as BIND-8.2.2-P5 in glibc in pre 2.3 release with
__RES == 19991006.

Signed-off-by: OBATA Akio <obache@wizdas.com>

src/network/qdns.cpp

@@ -102,7 +102,7 @@ static Q_UINT32 now()
     return 0;
 }
 
-#if defined(__GLIBC__) && ((__GLIBC__ > 2) || ((__GLIBC__ == 2) && (__GLIBC_MINOR__ >= 3)))
+#if defined(__RES) && (__RES >= 19980901)
 #define Q_MODERN_RES_API
 #else
 #endif