There is an updated pull request by tornaria against master on the void-packages repository

https://github.com/tornaria/void-packages giac
https://github.com/void-linux/void-packages/pull/45735

giac: fix `#include <giac/giac.h>` in musl
As reported in https://github.com/void-linux/void-packages/pull/45708#issuecomment-1688333569, including <giac/giac.h> fails in musl because the HAVE_TGAMMAF macro from autotools is not available at this time.

Here we include a patch adapted from https://github.com/geogebra/giac/commit/618a5de3349f8def5bdb909fea42fb447b6aad4a which is the solution upstream will include in next release of giac.

We need to fix this now so we can build sagemath in musl!

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A patch file from https://github.com/void-linux/void-packages/pull/45735.patch is attached