Fix definition check for GNUC
diff --git a/include/json/config.h b/include/json/config.h
index 8724ad9..2f70522 100644
--- a/include/json/config.h
+++ b/include/json/config.h
@@ -108,7 +108,7 @@
#if __has_extension(attribute_deprecated_with_message)
#define JSONCPP_DEPRECATED(message) __attribute__((deprecated(message)))
#endif
-#elif defined __GNUC__ // not clang (gcc comes later since clang emulates gcc)
+#elif defined(__GNUC__) // not clang (gcc comes later since clang emulates gcc)
#if (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5))
#define JSONCPP_DEPRECATED(message) __attribute__((deprecated(message)))
#elif (__GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 1))
@@ -123,7 +123,7 @@
#define JSONCPP_DEPRECATED(message)
#endif // if !defined(JSONCPP_DEPRECATED)
-#if __GNUC__ >= 6
+#if defined(__GNUC__) && (__GNUC__ >= 6)
#define JSON_USE_INT64_DOUBLE_CONVERSION 1
#endif