verify: simplify static_assert configuration * lib/verify.h (_GL_HAVE__STATIC_ASSERT, _GL_HAVE__STATIC_ASSERT1): Do not define for C++. This should be good enough nowadays, since recent-enough C++ compilers have static_assert. (_GL_HAVE_STATIC_ASSERT_CXX11, _GL_HAVE_STATIC_ASSERT_CXX17): Remove. All uses replaced by simply checking __cpp_static_assert.