https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92484
--- Comment #5 from franz <franz.flasch at gmx dot at> --- OK, I've now digged a little further in to this issue. According to the GCC docs for a native build a 3-stage-build is performed automatically. So with "--disable-bootstrap" the build should behave the same as the crosscompile build. I've tested it and isl builds fine with this setting. I've checked config.log from isl and yes, c++11 flags are not added in this case. However I actually don't know why this even has any influence on that. It seems that in the newest isl version they now use the M4 macro "AX_CXX_COMPILE_STDCXX_11" for the whole build. In the previous versions it was only used for "interface/isl_test_cpp", that's the reason why this problem actually came up.
