It works now. Thanks a lot for the help. Details below.

Ammar Ali wrote in post #965991:
> Try:
> $ locate libffi

That showed me two files which, in my uninformed opinion, should have
been valid:
/usr/lib/libffi.so.5
/usr/local/lib64/libffi.so.4

> If it's not found, then either use your system's package manager to
> install it, or grab the source from http://sourceware.org/libffi/ and
> compile/install it yourself.

In Synaptic, I installed libffi-dev. Then I did 'sudo gem pristine ffi'.
Running demo_rubygame.rb still gave me the same error.

Then, I installed libffi from source. Still the same error.

Then, I remembered to do 'sudo gem pristine ffi', and now it *does*
work!

>> And why doesn't 'gem install' catch the error?
>
> The gem checks for the library (via pkg-config) and for the required
> development header. I'm not familiar with Trisquel, perhaps it uses
> different conventions that are not covered/tested by the gem.

Trisquel is based on Ubuntu. I do have a pkg-config command.

Thanks again...
Luther

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