require C++11 to generate bindings

require C++11 to generate bindings

C++11 has always been needed to use the C++ bindings,
but commit isl-0.21-8-g7fe7f37942 (C++ bindings: automatically
convert long arguments to isl::val, Thu Dec 27 14:45:28 2018 +0100)
inadvertently introduced C++11 constructs in the generating code.
While it would be possible to remove these constructs,
it would make the code significantly more verbose.

So, perhaps it is time to bite the bullet and just require C++11
to generate the bindings.
This means that users who are only interested in the Python bindings
now also need a C++11 compiler, but they already need the clang libraries
anyway, so they might as well use the corresponding clang binary too.
Even the oldest supported version of clang (2.9) supports C++11
(as -std=c++0x).

Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>