There's a merged pull request on the void-packages repository

libgsm: update to 1.0.19.
https://github.com/void-linux/void-packages/pull/35647

Description:
Old distfiles are apparently not deleted, so I changed them to upstream.
I also cleaned up the template a little bit and do the install using the Makefile target instead of doing this manually.
Until now, libgsm-devel contained unnecessary header files, because all header files were just copied over.
Also includes manual pages now, which were not installed before.

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