mumps: install mumps libseq headers to include/mumps_seq

Directory libseq contains dummy MPI/BLACS/ScaLAPACK symbols to allow
linking/running MUMPS on a platform where MPI is not installed.
The install directory name mumps_seq comes from Debian source package
and Aur pkgbuild. See
- https://sources.debian.org/src/mumps/5.7.3-2/debian/libmumps-headers-dev.install
- https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=mumps-seq

pkgs/by-name/mu/mumps/package.nix

@@ -47,9 +47,12 @@ stdenv.mkDerivation (finalAttrs: {
     mkdir $out
     cp -r include lib $out
 
+    # Install mumps_seq headers
+    install -Dm 444 -t $out/include/mumps_seq libseq/*.h
+
     # Add some compatibility with coin-or-mumps
     ln -s $out/include $out/include/mumps
-    cp libseq/mpi.h $out/include/mumps_mpi.h
+    ln -s $out/include/mumps_seq/mpi.h $out/include/mumps_mpi.h
   '';
 
   nativeBuildInputs = [ gfortran ];