Disable -Wplacement-new warning false alarm

GCC-6 adds a -Wplacement-new warning that warns for placement-new into a
space that is too small, which is sometimes being triggered here (e.g.
example5 always generates the warning under g++-6).  It's a false
warning, however: the line immediately before just checked the size, and
so this line is never going to actually be reached in the cases where
the GCC warning is being triggered.

This localizes the warning disabling just to this one spot as there are
other placement-new uses in pybind11 where this warning could warn about
legitimate future problems.

include/pybind11/pybind11.h

@@ -82,7 +82,14 @@ protected:
 
         /* Store the capture object directly in the function record if there is enough space */
         if (sizeof(capture) <= sizeof(rec->data)) {
+#if defined(__GNUG__) && !defined(__clang__)
+#  pragma GCC diagnostic push
+#  pragma GCC diagnostic ignored "-Wplacement-new"
+#endif
             new ((capture *) &rec->data) capture { std::forward<Func>(f) };
+#if defined(__GNUG__) && !defined(__clang__)
+#  pragma GCC diagnostic pop
+#endif
             if (!std::is_trivially_destructible<Func>::value)
                 rec->free_data = [](detail::function_record *r) { ((capture *) &r->data)->~capture(); };
         } else {