Fix CI by reverting to coq_makefile

[pi8027]

Let me run CI here since the multinomials package does not seem to compile with coq-master.

Closes #75
Closes #76

[pi8027]

Does anyone know how to fix this error? @gares @proux01

  # File "./src/xfinmap.v", line 2, characters 0-68:
  # Error: Cannot load elpi_elpi.dummy: no physical path bound to elpi_elpi

[proux01]

Unforunately, I fear this is a dune related issue and I have no clue what dune is doing wrong.
The easiest and fastest fix is probably to revert to coq_makefile here for now.

[pi8027]

Also, this patch breaks the build of multinomials with Coq <= 8.18...

[Alizter]

Does the version of elpi change between Coq versions? It seems strange that the coq.theory elpi_elpi is missing only for older versions?

[Alizter]

Looks like the version of coq-elpi is changing between the mathcomp versions. In 2.2.0 is requires the extra elpi_elpi theory that was added. I don't really have time to help hacking around this and dune doesn't really support the way elpi requires things to be built (nor do we have the manpower to improve this at the moment). So perhaps the best thing to do is revert to coq_makefile here.

[pi8027]

@proux01 @Alizter Thanks for the feedback.

Since I couldn't find a way to make the build of multinomials compatible both with Coq 8.18 and master, I'm going to revert it to coq_makefile now. CC: @strub.

[pi8027]

This PR should also close #75 and #76. @haansn08

[pi8027]

Since this PR is blocking the merge of coq-community/apery#24, I'm merging this now, but please let me know if you see any issues.