gfan: fix clang build

Internal logging macro shadows std's `log2`, so we rename it.

pkgs/applications/science/math/gfan/default.nix

@@ -14,6 +14,10 @@ stdenv.mkDerivation rec {
 
   postPatch = lib.optionalString stdenv.cc.isClang ''
     substituteInPlace Makefile --replace "-fno-guess-branch-probability" ""
+
+    for f in $(find -name "*.h" -or -name "*.cpp"); do
+        substituteInPlace "$f" --replace-quiet "log2" "_log2"
+    done
   '';
 
   buildFlags = [ "CC=${stdenv.cc.targetPrefix}cc" "CXX=${stdenv.cc.targetPrefix}c++" ];