avutil/attributes: add av_warn_unused_result

GCC 3.4 introduced an attribute warn_unused_result to warn when a programmer
discards the return value. Applying this judiciously across the codebase can help
in fixing a lot of problems. At a high level, functions which return error codes
should always be checked. More concretely, consider the functions ff_add_format
and the like in avfilter/formats.h. A quick examination shows that a large portion
of libavfilter fails to handle the associated errors, usually AVERROR(ENOMEM).
The above example was where I observed the utility of this, but it should be
useful in many places across the code base.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/attributes.h

@@ -52,6 +52,12 @@
 #endif
 #endif
 
+#if AV_GCC_VERSION_AT_LEAST(3,4)
+#    define av_warn_unused_result __attribute__((warn_unused_result))
+#else
+#    define av_warn_unused_result
+#endif
+
 #if AV_GCC_VERSION_AT_LEAST(3,1)
 #    define av_noinline __attribute__((noinline))
 #elif defined(_MSC_VER)