Explicitely state that polka depends on num

Indeed, otherwise, the compilation fails for ocaml 4.06 >= if it is the system switch.

.gitlab-ci.yml

@@ -2,7 +2,7 @@ image: ocaml/opam:ubuntu
 
 variables:
   GIT_STRATEGY: clone
-
+  
 stages:
   - build
   - test
@@ -27,3 +27,4 @@ test_opam_test:
     - opam update
     - opam depext  -y mlgmpidl lutin
     - opam install -y lutin
+

_oasis

 OASISFormat: 0.4
 Name:        Lutin
-Version:     2.55
+Version:     2.56
 Authors:     Erwan Jahier, Pascal Raymond, Bertrand Jeannnet (polka), Yvan Roux
 Maintainers: erwan.jahier@univ-grenoble-alpes.fr  
 License:     CeCILL
@@ -113,7 +113,7 @@ Library polka
   XMETADescription:  polka: convex polyhedron library by Bertrand Jeannet (now part of apron)
   Path:    polka
   Modules: Polka,PolkaIO,Vector,Matrix,Poly,Polka_parser,Polka_lexer
-  BuildDepends: gmp # actually it is named mlgmpidl in opam
+  BuildDepends: num,gmp # actually it is named mlgmpidl in opam
   Install: true
   CSources: pkint.h, config.h,
             polka.h, internal.h, bit.h, satmat.h, cherni.h,

_tags

 # OASIS_START
-# DO NOT EDIT (digest: 3085b54b9feb3f809be8464ac1e6da02)
+# DO NOT EDIT (digest: f91833327e6d3fab520dfafa80bcf99e)
 # Ignore VCS directories, you can use the same kind of rule outside
 # OASIS_START/STOP if you want to exclude directories that contains
 # useless stuff for the build process
@@ -56,18 +56,31 @@ true: annot, bin_annot
 <polka/polka.{cma,cmxa}>: oasis_library_polka_dlllib
 <polka/polka.{cma,cmxa}>: use_libpolka_stubs
 <polka/*.ml{,i,y}>: package(gmp)
+<polka/*.ml{,i,y}>: package(num)
 "polka/polka_c.c": package(gmp)
+"polka/polka_c.c": package(num)
 "polka/internal.c": package(gmp)
+"polka/internal.c": package(num)
 "polka/bit.c": package(gmp)
+"polka/bit.c": package(num)
 "polka/satmat.c": package(gmp)
+"polka/satmat.c": package(num)
 "polka/cherni.c": package(gmp)
+"polka/cherni.c": package(num)
 "polka/vector_c.c": package(gmp)
+"polka/vector_c.c": package(num)
 "polka/matrix_c.c": package(gmp)
+"polka/matrix_c.c": package(num)
 "polka/poly_c.c": package(gmp)
+"polka/poly_c.c": package(num)
 "polka/vector_caml.c": package(gmp)
+"polka/vector_caml.c": package(num)
 "polka/matrix_caml.c": package(gmp)
+"polka/matrix_caml.c": package(num)
 "polka/poly_caml.c": package(gmp)
+"polka/poly_caml.c": package(num)
 "polka/polka_caml.c": package(gmp)
+"polka/polka_caml.c": package(num)
 # Library lutin
 "lutin/src/lutin.cmxs": use_lutin
 "lutin/src/lutin.cmxa": oasis_library_lutin_dlllib

lutin/src/version.ml

-let str="2.55"
-let sha="e85ae63"
+let str="2.56"
+let sha="a7040a2"