[bug#38687] [PATCH] gnu: Add libtcod.

[goodoldpaul]

Hi Marius and Ludo,
I managed to remove all vendored libraries except for glad.h which seems 
to be some kind of generated glue code for loading OpenGL 
(https://github.com/Dav1dde/glad). In the next two patches I'm adding 
libtcod and it's dependency lodepng.

Guix lint is warning me that lodepng could be affected by 
CVE-2019-17178, but taking a look at 
https://nvd.nist.gov/vuln/detail/CVE-2019-17178 and 
https://nvd.nist.gov/vuln/search/results?adv_search=true&cpe_version=cpe%3a%2fa%3alodev%3alodepng%3a2019-09-28 
seems to indicate that lodepng should  be *not* vulnerable since 
28/09/2019, did I understand correctly?

Please don't hesitate and tell me if anything should done w.r.t. the 
CVE.

Thanks for your patience reviewing this,

Giacomo