The message on configure is misleading:
checking whether the linker accepts -std=c++11... yes
checking whether the linker accepts -std=c++14... yes
checking whether the linker accepts -std=c++17... no
It is the compiler that we check, not just the linker.
* m4/ax_check_link_flag.m4: Remove.
* m4/bison-check-compiler-flag.m4: New.
* m4/bison-cxx-std.m4: Use it.
