isl 0.26 C++17 issues when bootstrapping with GCC

[Ben Boeckel via Gcc]

Hi,

I tried adding isl 0.26 to a 13.2 GCC build using Iain's macOS aarch64
patches:

    https://github.com/iains/gcc-13-branch

It seems that the bootstrap's `CXX='g++ -std=c++11'` confuses isl's
build where C++17 is expected to work by disabling C++17 behind its
back. Should GCC not add this flag for its dependencies but only its own
build perhaps?

isl bug report thread (with isl configure logs and the like):

    https://groups.google.com/g/isl-development/c/ShnQcW_35ZQ

Thanks,

--Ben