Remove chained targets in c_src/Makefile to avoid rebuild failures

There was a bug in the `c_src/Makefile` that caused the decompression and
renaming of the directory in the tar file with the C dependencies to happen
every time the project was built. The bug was experienced as a build
failure when recompiling for the third time. The error that was shown was:
```
mv: cannot move ‘zeromq-3.2.2’ to ‘zeromq3/zeromq-3.2.2’: Directory not empty
```
This was happening because the untarred directory's (`zeromq3`) date in the
filesystem was older than the one from the tar file (`zeromq-3.2.2.tar.gz`).
This was discovered by debugging the make process and seeing this message:
```
Prerequisite `../deps/zeromq-3.2.2.tar.gz' is newer than target `../deps/zeromq3'.
Must remake target `../deps/zeromq3'.
```
The problem was fixed by avoiding chained targets in the `Makefile`.