Adding license: GPL-3

Adding license file (downloaded from gnu.org) for GPL-3.
Added file 'AUTHORS' with a list of participating authors
that hold the copyright for this software under the GPL-3.
Added a comment on top of the single C source file stating
that this software is licensed under the GNU General Public
License version 3 or any later version.
Out-of-tree builds; refactoring into multiple files

The Makefile will not advise the C compiler to write the
object files into a temporary directory inside /tmp/.
This is meant and beneficial if /tmp is a RAM disk (fast)
and storage location of the C code is a SSD (don't like
many write operations).

The previously single source file was refactored: Several
generic helper functions reside now in utils.c and can be
accessed by including utils.h.