intention to unretire log4cxx

[Till Hofmann]

Hi all,

I intend to unretire log4cxx, which was retired because it was orphaned
for too long. Among others, log4cxx is used by the Robot Operating
System ROS [1] and thus is quite important for robotics applications in
Fedora.

I've updated the spec file to build with GCC6, moved the documentation
to a noarch subpackage, and I did some general spec cleanup.

I've submitted a review request:
https://bugzilla.redhat.com/show_bug.cgi?id=1408428

Kind regards,
Till

[1] http://www.ros.org/