protobuf-c: always define PROTOBUF_C__DEPRECATED

If we are not building on gcc, PROTOBUF_C__DEPRECATED needs to be
defined but empty. Previously we were not defining it at all in certain
situations, which would cause build failures.

Based on a patch from alex85k (#167).

protobuf-c/protobuf-c.h

@@ -205,10 +205,9 @@ PROTOBUF_C__BEGIN_DECLS
 # define PROTOBUF_C__API
 #endif
 
-#if !defined(PROTOBUF_C__NO_DEPRECATED)
-# if (__GNUC__ > 3) || (__GNUC__ == 3 && __GNUC_MINOR__ >= 1)
+#if !defined(PROTOBUF_C__NO_DEPRECATED) && \
+	((__GNUC__ > 3) || (__GNUC__ == 3 && __GNUC_MINOR__ >= 1))
 # define PROTOBUF_C__DEPRECATED __attribute__((__deprecated__))
-# endif
 #else
 # define PROTOBUF_C__DEPRECATED
 #endif