openblas: fix config breakage introduced for configs using 'false'

My earlier change mistakenly expected `toString false` to produce '0'
instead of the empty string, leading to unexpected config changes.

Intended to address issue mentioned here and in following discussion:

https://github.com/NixOS/nixpkgs/pull/53972#issuecomment-459981602

Sorry, folks!

(special-case handling of bools here makes this "cleanup" a bit
 less of an obvious win but hopefully still preferable overall :))

-----------

makeFlags in resulting derivation, according to this one-liner:

$ nix show-derivation -f . openblas|jq ".[].env.makeFlags"

before:
"BINARY=64 CC=cc CROSS= DYNAMIC_ARCH=1 FC=gfortran HOSTCC=cc INTERFACE64=1 NO_BINARY_MODE= NO_STATIC=1 NUM_THREADS=64 PREFIX=/1rz4g4znpzjwh1xymhjpm42vipw92pr73vdgl6xs1hycac8kf2n9 TARGET=ATHLON USE_OPENMP=1"

after:
"BINARY=64 CC=cc CROSS=0 DYNAMIC_ARCH=1 FC=gfortran HOSTCC=cc INTERFACE64=1 NO_BINARY_MODE=0 NO_STATIC=1 NUM_THREADS=64 PREFIX=/1rz4g4znpzjwh1xymhjpm42vipw92pr73vdgl6xs1hycac8kf2n9 TARGET=ATHLON USE_OPENMP=1"

Without knowing how `placeholder` works, it seems interesting if
entirely unrelated that the `PREFIX` is same for both! :). TIL.

1 files changed, 9 insertions, 1 deletions

diff --git a/pkgs/development/libraries/science/math/openblas/default.nix b/pkgs/development/libraries/science/math/openblas/default.nix
index 68439f5921d8..f4a97b7cf854 100644
--- a/pkgs/development/libraries/science/math/openblas/default.nix
+++ b/pkgs/development/libraries/science/math/openblas/default.nix
@@ -74,6 +74,14 @@ let
     if blas64_ != null
       then blas64_
       else hasPrefix "x86_64" stdenv.hostPlatform.system;
+  # Convert flag values to format OpenBLAS's build expects.
+  # `toString` is almost what we need other than bools,
+  # which we need to map {true -> 1, false -> 0}
+  # (`toString` produces empty string `""` for false instead of `0`)
+  mkMakeFlagValue = val:
+    if !builtins.isBool val then toString val
+    else if val then "1" else "0";
+  mkMakeFlagsFromConfig = mapAttrsToList (var: val: "${var}=${mkMakeFlagValue val}");
 in
 stdenv.mkDerivation rec {
   name = "openblas-${version}";
@@ -109,7 +117,7 @@ stdenv.mkDerivation rec {
     buildPackages.stdenv.cc
   ];
 
-  makeFlags = mapAttrsToList (var: val: "${var}=${toString val}") (config // {
+  makeFlags = mkMakeFlagsFromConfig (config // {
     FC = "${stdenv.cc.targetPrefix}gfortran";
     CC = "${stdenv.cc.targetPrefix}${if stdenv.cc.isClang then "clang" else "cc"}";
     PREFIX = placeholder "out";