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 fix by avoiding chained targets in the Makefile.
Merge pull request #59 from jcomellas/jc-unchain-c_src-make-targets

Remove chained targets in c_src/Makefile to avoid rebuild failures