armadillo: Make use of openBLAS's built-in LAPACK

Even though the linked openBLAS contains LAPACK, the functionality was
not usable due to armadillo needing to link against it explicitly.

pkgs/development/libraries/armadillo/default.nix

···
   nativeBuildInputs = [ cmake ];
   buildInputs = [ openblasCompat superlu hdf5 ];
 
-  cmakeFlags = [ "-DDETECT_HDF5=ON" ];
+  cmakeFlags = let
+    libSuff = if stdenv.isDarwin then "dylib" else "so";
+  in [
+    "-DLAPACK_LIBRARY=${openblasCompat}/lib/libopenblas.${libSuff}"
+    "-DDETECT_HDF5=ON"
+  ];
 
- patches = [ ./use-unix-config-on-OS-X.patch ];
+  patches = [ ./use-unix-config-on-OS-X.patch ];
 
   meta = with stdenv.lib; {
     description = "C++ linear algebra library";