[Bug libstdc++/79702] AX_CXX_COMPILE_STDCXX([17]) does not work with GCC=7.0.1

[trippels at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79702

--- Comment #2 from Markus Trippelsdorf <trippels at gcc dot gnu.org> ---
And #include <tuple> is enough to reproduce.