Description
Apery is under the CeCILL-C license. This is a weak-copyleft license from the CeCILL family. While the basic CeCILL (v2) license is GPL-compatible, CeCILL-C suffers from the same issues as CeCILL-B, i.e., it is not GPL-compatible. This is usually considered a serious flaw for an open source license and should discourage its use.
While a license change is usually a very difficult process because it requires contacting all the copyright holders (all the contributors), it appears that in the case of Apery, that number is still small enough to make this process doable.
If keeping the CeCILL-C license is deemed important, a standard solution is to dual license the code. This means that there are two licenses that apply to it, and anyone is free to choose which of the two licenses they have to abide by.
To preserve the spirit of the CeCILL-C licence, I propose to use the MPL 2.0 license as a replacement or additional license. This is one of the two licenses recommended in Coq-community, and it is the closest widely used and GPL-compatible license that has conditions similar to the CeCILL-C.