Enable building with system hiredis

[orgads]

No description provided.

[davehorton]

This was clearly not tested with a local hiredis install. In my testing if fails to build.

/usr/bin/ld: /home/runner/work/drachtio-server/drachtio-server/build/../src/blacklist.cpp:216: undefined reference to `redisFree'
/usr/bin/ld: src/drachtio-blacklist.o: in function `drachtio::QuerySentinel(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, boost::asio::ip::basic_endpoint<boost::asio::ip::tcp> const&, std::unordered_set<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::hash<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::equal_to<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >&)':
/home/runner/work/drachtio-server/drachtio-server/build/../src/blacklist.cpp:76: undefined reference to `redisConnect'

[orgads]

It was tested on docker. I'll test it further.

[orgads]

Sorry, I cherry-picked it from my branch with the wrong base. Should be ok now.

[davehorton]

@orgads
just so I understand, is your intent for this PR that drachtio server only builds with system hiredis, or that it builds with either (system provided or deps provided)?

[orgads]

Both work. The previous revision had -d instead of -f in the condition, so it failed when hiredis was not installed on the system. I corrected that.

[orgads]

Rebased (based on "Improve docker" PR).