removed default dependency of liblapack on ATLAS. was causing an unnecessary double-build of atlas

pkgs/development/libraries/science/math/liblapack/default.nix

@@ -1,15 +1,38 @@
-{ stdenv, fetchurl, gfortran, atlas, cmake, python, shared ? false }:
+{
+  stdenv,
+  fetchurl,
+  gfortran,
+  cmake,
+  python,
+  atlas ? null,
+  shared ? false,
+  version ? "3.4.1"
+}:
 let
-  atlasMaybeShared = atlas.override { inherit shared; };
+  atlasMaybeShared = if atlas != null then atlas.override { inherit shared; }
+                     else null;
   usedLibExtension = if shared then ".so" else ".a";
-  version = "3.4.1";
-  inherit (stdenv.lib) optional;
+  inherit (stdenv.lib) optional optionals concatStringsSep;
+  inherit (builtins) hasAttr attrNames;
+  # Hashes of the versions of liblapack we know about.
+  versions2sha = {
+    "3.4.1" = "93b910f94f6091a2e71b59809c4db4a14655db527cfc5821ade2e8c8ab75380f";
+    "3.5.0" = "0lk3f97i9imqascnlf6wr5mjpyxqcdj73pgj97dj2mgvyg9z1n4s";
+  };
 in
+
+if !(builtins.hasAttr version versions2sha)
+then throw ''
+  Unknown liblapack version ${version}.
+  Available versions: ${concatStringsSep ", " (attrNames versions2sha)}
+''
+else
+
 stdenv.mkDerivation rec {
   name = "liblapack-${version}";
   src = fetchurl {
     url = "http://www.netlib.org/lapack/lapack-${version}.tgz";
-    sha256 = "93b910f94f6091a2e71b59809c4db4a14655db527cfc5821ade2e8c8ab75380f";
+    sha256 = versions2sha."${version}";
   };
 
   propagatedBuildInputs = [ atlasMaybeShared ];
@@ -18,10 +41,12 @@ stdenv.mkDerivation rec {
 
   cmakeFlags = [
     "-DUSE_OPTIMIZED_BLAS=ON"
-    "-DBLAS_ATLAS_f77blas_LIBRARY=${atlasMaybeShared}/lib/libf77blas${usedLibExtension}"
-    "-DBLAS_ATLAS_atlas_LIBRARY=${atlasMaybeShared}/lib/libatlas${usedLibExtension}"
     "-DCMAKE_Fortran_FLAGS=-fPIC"
   ]
+  ++ (optionals (atlas != null) [
+    "-DBLAS_ATLAS_f77blas_LIBRARY=${atlasMaybeShared}/lib/libf77blas${usedLibExtension}"
+    "-DBLAS_ATLAS_atlas_LIBRARY=${atlasMaybeShared}/lib/libatlas${usedLibExtension}"
+  ])
   ++ (optional shared "-DBUILD_SHARED_LIBS=ON")
   # If we're on darwin, CMake will automatically detect impure paths. This switch
   # prevents that.