bug#34854: stepmania does not work

[Ricardo Wurmus]

“stepmania” cannot find its Songs directory, so I created it.  After
doing that, however, “stepmania” still cannot be started as it can’t
find GtkModule.so.

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$ /gnu/store/awp5w5v3hfg8zilviarwj85s2y820lyg-stepmania-5.1.0-b2/bin/stepmania 
--help

//////////////////////////////////////////////////////
Exception: Couldn't find 'Songs'
//////////////////////////////////////////////////////

Error: Couldn't find 'Songs'

$ mkdir Songs
$ /gnu/store/awp5w5v3hfg8zilviarwj85s2y820lyg-stepmania-5.1.0-b2/bin/stepmania 
StepMania5.1-UNKNOWN
Compiled 19700101 @ 00:00:01 (build UNKNOWN)
Log starting 2019-03-14 09:15:27
Couldn't load driver gtk: dlopen(): 
/gnu/store/awp5w5v3hfg8zilviarwj85s2y820lyg-stepmania-5.1.0-b2/bin/GtkModule.so:
 cannot open shared object file: No such file or directory
Error: Couldn't open any loading windows.
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-- 
Ricardo