Update rdkafka_msg.c

gcc version 4.1.2 20080704 (Red Hat 4.1.2-44)

$[home/kafka/librdkafka-master] make all
gcc -MD -MP -g -O2 -fPIC -Wall -Werror -Wfloat-equal -Wpointer-arith  -c rdkafka_msg.c -o rdkafka_msg.o
cc1: warnings being treated as errors
rdkafka_msg.c: In function 'rd_kafka_msg_new':
rdkafka_msg.c:137: warning: value computed is not used
make: *** [rdkafka_msg.o] Error 1

rdkafka_msg.c:137 : 'rd_atomic_sub(&rkt->rkt_rk->rk_producer.msg_cnt, 1);' and '(void)'
Merge pull request #127 from liyujun/patch-1

Fix gcc 4.1 compiler warning: value not used (patch from liyujun)
Initialize rkb_lock
Topics were not destroyed in example code (issue #130)
WIP: Added consumer queue rerouting

This allows the application to consume from multiple topic+partitions
with a single rd_kafka_consume*() call, instead of one such
call per topic+partition.
Added consumer queue rerouting

This allows the application to consume from multiple topic+partitions
with a single rd_kafka_consume*() call, instead of one such
call per topic+partition.
Added "unique" test ids for message validation
Added RD_KAFKA_OFFSET_TAIL(cnt) to start consuming 'cnt' messages from end.

An application can now easily consume the last 'cnt' messages from a partition.
Added produce->consume test (produce batch, consumer queues, TAIL)
produce_batch(): Fixed race condition when cluster comes up

If rdkafka is connecting to the cluster for the first time at the same
time as a larger batch is produced the message batch may end up
stuck in the Unassigned partition queue.
Allow Conf object reuse in C++ interface
Initialize Conf object members (issue #125)
Protect from corrupt metadata (unknown partitions, issue #132)
RD_KAFKA_OFFSET_TAIL() no longer messes up Offset requests (issue #136)
Fixed SYMDUMPER on OSX
Use .dylib extension on OSX
0.8.4 versioning