Unix packages (tar, rpm, deb) & some Unix installation fixes

[thomaslepoix]

Hi! Deep respect for your work on doom64. Some things in this PR :

• Avoid propagating make install to fluidsynth as deploying fluidsynth headers (/usr[/local]/include/fluidsynth*) is not required by doom64ex and it could eventually conflicts with distro versions.

• Install doom64ex in /usr[/local]/games instead of /usr[/local]/bin on unix targets.

• Install icon, manpage and desktop file on unix targets.

• Implement make package on unix targets, that creates tar archives, deb and rpm packages. (Issue #48)

I work with Ubuntu 18.04, the deb package seems to be ok, the rpm seems to be quite identical but I have not tested it.

[pinkwah]

This looks pretty good to me. I'll wait for @svkaiser to look at this since I haven't touched the project in ages.