setup.py: Fallback to use the system hiredis library.

[apteryks]

Fixes #158 fully, including using a system-provided hiredis.

When the hiredis git submodule hasn't been initialized, print a message about it, and attempt to link against the a system-provided hiredis library instead.

• setup.py (is_hiredis_bundled): New procedure. (get_hiredis_bundled_sources): Likewise. Print a message when bundled_hiredis_sources is empty. (get_sources): Adjust to use the above procedure. (get_linker_args): Add -lhiredis when the bundled hiredis is not used.