bug#64307: Fluxbox build fails after e6994d7

[Cairn]

Commit e6994d7 causes Fluxbox to fail on my aarch64 system. Might disturb other 
targets as well, but I can't test right now. just to be sure, here's the commit 
immediately prior to it, which succeeds:

    guix time-machine --commit=74e96c4 -- build fluxbox

The trouble commit deals with applying a fix to `gcc-4.9`, which helped me find 
that Fluxbox will succeed with that particular version:

    guix build --with-c-toolchain=fluxbox=gcc-toolchain@4.9 fluxbox

Would it be acceptable to update the Fluxbox definition to use that version? 
Although maybe there's a deeper issue here that I'm missing. Let me know!

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