configure.ac: Better hiredis detection

Instead of relying on pkg-config to detect hiredis (which is specific to
Debian and derived distributions), search for hiredis/hiredis.h on the
default include path too.

Also corrected the --with-libhiredis= case too, as it was checking for
<hiredis.h> alone, instead of <hiredis/hiredis.h>.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>