gnu: openttd: Include openttd-openmsx and openttd-opensfx.

gnu: openttd: Include openttd-openmsx and openttd-opensfx.

* gnu/packages/games.scm (openttd-opengfx)[arguments]: Change installation
directory from /share/openttd/baseset/opengfx to
/share/games/openttd/baseset/opengfx.
(openttd-engine)[arguments]: Support #:configure-flags keyword argument in
'configure' phase.
(openttd)[inputs]: Add timidity++.
[native-inputs]: Add openttd-openmsx and openttd-opensfx.
[arguments]: Configure with timidity as MIDI player. Install data from
openttd-openmsx and openttd-opensfx.

Signed-off-by: Kei Kebreau <kkebreau@posteo.net>