Allow to build the GNU/Hurd implementation

The possibility to build the GNU/Hurd implementation has not been introduced when switching from autotools to a plain Makefile;
hence add the sources for GNU/Hurd when PLATFORM=gnu
Update READMEs about GNU/Hurd

Mention the "gnu" platform in the main README.
Add a very short README.gnu to specify it is for GNU/Hurd, and credit Samuel.
Merge pull request #2 from pinotree/hurd-build

Allow to build the GNU/Hurd implementation