verify: Use C++11 static_assert when available. Reported by Alexandre Duret-Lutz <adl@lrde.epita.fr> in <https://lists.gnu.org/archive/html/bug-gnulib/2021-01/msg00177.html>. * lib/verify.h (_GL_HAVE_STATIC_ASSERT_CXX11): New macro. (_GL_HAVE_STATIC_ASSERT_CXX17): Renamed from _GL_HAVE_STATIC_ASSERT1. (_GL_VERIFY): Use static_assert when available with C++11 syntax.