Add pkg-config info for the riak-c-client

Tested using a basic build:
```
./autogen.sh && ./configure && make && sudo make install
```

```
$ pkg-config --cflags --libs riak-c-client-sync
-lriak_c_client-0.5

$ pkg-config --cflags --libs riak-c-client-async
-D_THREAD_SAFE -I/usr/local/Cellar/libevent/2.0.21/include -L/usr/local/Cellar/libevent/2.0.21/lib -lriak_c_client-0.5 -levent_pthreads -levent
```