Fix builds on old CMake versions

On older cmake releases (like the one shipped with CentOS 6) the build process fails to find `avro-c.pc` file during `make install`. This patch makes sure the file is in the right place by moving it to `src` directory before performing `make install` step (it only does that when the file is in the wrong place).

For more information about the issue, please check out [AVRO-1222](https://issues.apache.org/jira/browse/AVRO-1222)