arpack: Enable eigenvalue-problems solver based on ICB and eigen

The arpack package included the eigen library and added it to the build
inputs but neglected to enable its use by setting `EIGEN=ON` in the
build flags.  Enable support for eigenvalue-problems solver based on ICB
and eigen and disable parallel checking as the tests fail when run in
parallel.

pkgs/by-name/ar/arpack/package.nix

@@ -30,10 +30,14 @@ stdenv.mkDerivation rec {
 
   nativeCheckInputs = lib.optional useMpi openssh;
 
+  # a couple tests fail when run in parallel
   doCheck = true;
+  enableParallelChecking = false;
 
   cmakeFlags = [
     (lib.cmakeBool "BUILD_SHARED_LIBS" stdenv.hostPlatform.hasSharedLibraries)
+    (lib.cmakeBool "EIGEN" true)
+    (lib.cmakeBool "EXAMPLES" true)
     (lib.cmakeBool "ICB" true)
     (lib.cmakeBool "INTERFACE64" blas.isILP64)
     (lib.cmakeBool "MPI" useMpi)
@@ -43,11 +47,6 @@ stdenv.mkDerivation rec {
     "-DBLA_VENDOR=Generic"
   ];
 
-  preCheck = ''
-    # Prevent tests from using all cores
-    export OMP_NUM_THREADS=2
-  '';
-
   passthru = {
     inherit (blas) isILP64;
     tests = {