Default-defined method on extern-templated class not visible (flexibl…

…e-collision-library#369)

This addresses a bug unique to versions of GCC prior to 6.3. It loses
methods that are `= default` defined.

include/fcl/geometry/shape/convex.h

@@ -102,7 +102,7 @@ class FCL_EXPORT Convex : public ShapeBase<S_>
          int num_faces, const std::shared_ptr<const std::vector<int>>& faces);
 
   /// @brief Copy constructor 
-  Convex(const Convex& other) = default;
+  Convex(const Convex& other);
 
   ~Convex() = default;
 
@@ -165,6 +165,12 @@ class FCL_EXPORT Convex : public ShapeBase<S_>
   Vector3<S> interior_point_;
 };
 
+// Workaround for https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57728 which
+// should be moved back into the class definition once we no longer need to
+// support GCC versions prior to 6.3.
+template <typename S>
+Convex<S>::Convex(const Convex<S>& other) = default;
+
 using Convexf = Convex<float>;
 using Convexd = Convex<double>;