isl 0.22 build error with GCC 9.2.0

[StephanTLavavej]

isl 0.22 is including <type_traits>, but apparently isn't being compiled with -std=c++11 or higher.

In file included from c:\mingw\include\c++\8.2.0\type_traits:35,
                 from ../../src/isl/include/isl/cpp.h:34,
                 from ../../src/isl/isl_test_cpp.cc:16:
c:\mingw\include\c++\8.2.0\bits\c++0x_warning.h:32:2: error: #error This file requires compiler and library support for the ISO C++ 2011 standard. This support must be enabled with the -std=c++11 or -std=gnu++11 compiler options.
 #error This file requires compiler and library support \
  ^~~~~
In file included from ../../src/isl/isl_test_cpp.cc:16:
../../src/isl/include/isl/cpp.h: In member function 'isl_ctx* isl::ctx::release()':
../../src/isl/include/isl/cpp.h:57:8: error: 'tmp' does not name a type; did you mean 'tm'?
   auto tmp = ptr;
        ^~~
        tm
../../src/isl/include/isl/cpp.h:58:9: error: 'nullptr' was not declared in this scope
   ptr = nullptr;
         ^~~~~~~

(I'm bootstrapping GCC 9.2.0 starting with GCC 8.2.0, hence the version in the error message.)

[StephanTLavavej]

Tracked by GCC-92484.