Add mpich overlay

m/common/main.nix

@@ -9,6 +9,8 @@
     ./slurm.nix
     ./ssh.nix
     ./users.nix
+
+    ../../pkgs/mpi.nix
   ];
 
   nixpkgs.overlays = [ bscpkgs.bscOverlay ];

pkgs/mpi.nix

@@ -0,0 +1,28 @@
+let
+  overlay = final: prev:
+  {
+    mpich = with final; prev.mpich.overrideAttrs (old: {
+      buildInput = old.buildInputs ++ [
+        libfabric
+        pmix
+      ];
+      configureFlags = [
+        "--enable-shared"
+        "--enable-sharedlib"
+        "--with-pm=no"
+        "--with-device=ch4:ofi"
+        "--with-pmi=pmix"
+        "--with-pmix=${final.pmix}"
+        "--with-libfabric=${final.libfabric}"
+        "--enable-g=log"
+      ] ++ lib.optionals (lib.versionAtLeast gfortran.version "10") [
+        "FFLAGS=-fallow-argument-mismatch" # https://github.com/pmodels/mpich/issues/4300
+        "FCFLAGS=-fallow-argument-mismatch"
+      ];
+    });
+  };
+in
+
+{
+  nixpkgs.overlays = [ overlay ];
+}