Add a new USE_SYSTEM_HIREDIS make variable to select whether to link
against the system libhiredis (and libhiredis_ssl if BUILD_TLS is
enabled).
Move the sdscompat.h header from the vendored hiredis directory to src/,
as this file is not and has never been part of the upstream hiredis
project, it got added in commit bffbbeaa9a1a6b8e81384297272cb0631502e8fd
in redis itself.
