atlas: allow users to override the CPU configuration used to build ATLAS

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

@@ -1,4 +1,24 @@
-{ stdenv, fetchurl, gfortran, tolerateCpuTimingInaccuracy ? true, shared ? false }:
+{ stdenv, fetchurl, gfortran, tolerateCpuTimingInaccuracy ? true, shared ? false
+, cpuConfig ? if stdenv.isi686 then "-b 32 -A 18 -V 1" else "-b 64 -A 31 -V 192"
+}:
+
+# Atlas detects the CPU and optimizes its build accordingly. This is great when
+# the code is run on the same machine that built the binary, but in case of a
+# central build farm like Hydra, this feature is dangerous because the code may
+# be generated utilizing fancy features that users who download the binary
+# cannot execute.
+#
+# To avoid these issues, the build is configured using the 'cpuConfig'
+# parameter as follows:
+#
+#   | x86 CPU                                     | x86_64 CPU             |
+#   |---------------------------------------------+------------------------|
+#   | -b 32                                       | -b 64                  |
+#   | -A 18  (Pentium II)                         | -A 31 (Athlon K7)      |
+#   | -V 1 (No SIMD: Pentium II doesn't have SSE) | -V 192 (SSE1 and SSE2) |
+#
+# Users who want to compile a highly optimized version of ATLAS that's suitable
+# for their local machine can override these settings accordingly.
 
 let
   version = "3.10.1";
@@ -15,6 +35,8 @@ stdenv.mkDerivation {
     sha256 = "11ncgdc7kzb2y2gqb3sgarm5saj9fr07r3h2yh2h5bja429b85d2";
   };
 
+  buildInputs = [ gfortran ];
+
   # Atlas aborts the build if it detects that some kind of CPU frequency
   # scaling is active on the build machine because that feature offsets the
   # performance timings. We ignore that check, however, because with binaries
@@ -25,30 +47,19 @@ stdenv.mkDerivation {
   # Configure outside of the source directory.
   preConfigure = '' mkdir build; cd build; configureScript=../configure; '';
 
-  # * -fPIC allows to build atlas inside shared objects, as octave does.
+  # * -fPIC is passed even in non-shared builds so that the ATLAS code can be
+  #   used to inside of shared libraries, like Octave does.
   #
-  # * Atlas detects the cpu and does some tricks. For example, notices the
+  # * -t 0 disables use of multi-threading. It's not quite clear what the
-  #   hydra AMD Family 10h computer, and uses a SSE trick for it (bit 17 of MXCSR)
+  #   consequences of that setting are and whether it's necessary or not.
-  #   available, for what I know, only in that family. So we hardcode K7
+  configureFlags = "-Fa alg -fPIC -t 0 ${cpuConfig}" + optionalString shared " --shared";
-  #     -A 31 = Athlon K7
-  #     -A 18 = Pentium II
-  #     -V 192 = SSE1|SSE2 (Or it takes SSE3 somehow in my machine without SSE3)
-  #     -V 1 = No SIMD (Pentium II does not have any SSE)
-  #     -t 0 = No threading
-  configureFlags = "-Fa alg -fPIC -t 0"
-    + optionalString stdenv.isi686 " -b 32 -A 18 -V 1"
-    + optionalString stdenv.isx86_64 " -A 31 -V 192"
-    + optionalString shared " --shared "
-    ;
-
-  buildInputs = [ gfortran ];
 
   doCheck = true;
 
   meta = {
     homepage = "http://math-atlas.sourceforge.net/";
     description = "Automatically Tuned Linear Algebra Software (ATLAS)";
-    license = "GPL";
+    license = stdenv.lib.licenses.bsd3;
 
     longDescription = ''
       The ATLAS (Automatically Tuned Linear Algebra Software) project is an ongoing