Fix usage of is_trivially_copyable on older GCC

src/libslic3r/Fill/FillBase.hpp

@@ -40,7 +40,11 @@ struct FillParams
     // in this case we don't try to make more continuous paths
     bool        complete;
 };
+#if (!defined __GNUC__) || __GNUC__ > 4
+// Older GCCs don't have std::is_trivially_copyable
+// cf. https://gcc.gnu.org/onlinedocs/gcc-4.9.4/libstdc++/manual/manual/status.html#status.iso.2011
 static_assert(std::is_trivially_copyable<FillParams>::value, "FillParams class is not POD (and it should be - see constructor).");
+#endif
 
 class Fill
 {