work around ADOL-C's comparison operators returning an `int`

Reference: https://gitlab.com/adol-c/adol-c/issues/18

.gitlab-ci.yml

 ---
 # Install external dependencies
 before_script:
+  - patch -d /duneci/modules/dune-common -p1 < dune-common-densematrix-adolc-workaround.diff
   - duneci-install-module https://git.imp.fu-berlin.de/agnumpde/dune-matrix-vector.git
   - duneci-install-module https://git.imp.fu-berlin.de/agnumpde/dune-fufem.git
   - duneci-install-module https://git.imp.fu-berlin.de/agnumpde/dune-solvers.git

dune-common-densematrix-adolc-workaround.diff

+diff --git a/dune/common/densematrix.hh b/dune/common/densematrix.hh
+index b03bbb0b..917ecef9 100644
+--- a/dune/common/densematrix.hh
++++ b/dune/common/densematrix.hh
+@@ -897,7 +897,7 @@ namespace Dune
+         for (size_type k=i+1; k<A.rows(); k++)
+         {
+           auto abs = fvmeta::absreal(A[k][i]);
+-          auto mask = abs > pivmax;
++          bool mask = abs > pivmax;
+           pivmax = Simd::cond(mask, abs, pivmax);
+           imax   = Simd::cond(mask, simd_index_type(k), imax);
+         }