slic3r: fix build with gcc6

pkgs/applications/misc/slic3r/default.nix

···
     sha256 = "1z8h11k29b7z49z5k8ikyfiijyycy1q3krlzi8hfd0vdybvymw21";
   };
 
+  patches = [
+    ./gcc6.patch
+  ];
+
   buildInputs = with perlPackages; [ perl makeWrapper which
     EncodeLocale MathClipper ExtUtilsXSpp threads
     MathConvexHullMonotoneChain MathGeometryVoronoi MathPlanePath Moo
···
 
   buildPhase = ''
     export SLIC3R_NO_AUTO=true
+    export LD=$CXX
     export PERL5LIB="./xs/blib/arch/:./xs/blib/lib:$PERL5LIB"
 
     substituteInPlace Build.PL \

pkgs/applications/misc/slic3r/gcc6.patch

···
+diff --git i/xs/src/libslic3r/Config.hpp w/xs/src/libslic3r/Config.hpp
+index 49e999b..d9b65d8 100644
+--- i/xs/src/libslic3r/Config.hpp
++++ w/xs/src/libslic3r/Config.hpp
+@@ -65,7 +65,7 @@ class ConfigOptionFloat : public ConfigOption
+     
+     bool deserialize(std::string str) {
+         std::istringstream iss(str);
+-        return iss >> this->value;
++        return bool(iss >> this->value);
+     };
+ };
+ 
+@@ -124,7 +124,7 @@ class ConfigOptionInt : public ConfigOption
+     
+     bool deserialize(std::string str) {
+         std::istringstream iss(str);
+-        return iss >> this->value;
++        return bool(iss >> this->value);
+     };
+ };
+ 
+@@ -249,7 +249,7 @@ class ConfigOptionPercent : public ConfigOption
+     bool deserialize(std::string str) {
+         // don't try to parse the trailing % since it's optional
+         std::istringstream iss(str);
+-        return iss >> this->value;
++        return bool(iss >> this->value);
+     };
+ };
+ 
+@@ -279,7 +279,7 @@ class ConfigOptionFloatOrPercent : public ConfigOption
+     bool deserialize(std::string str) {
+         this->percent = str.find_first_of("%") != std::string::npos;
+         std::istringstream iss(str);
+-        return iss >> this->value;
++        return bool(iss >> this->value);
+     };
+ };
+