- configure.ac: Replace check for C++11 with C++17 --
C++17 is already required when building the Qt6 bindings, this change
make this also a requirement when building the pure c++ and qt5
C++17 is quite mature nowadays and is implemented by all major compilers
and is the default C++ version used by GCC and clang. This will allow us
to use some slightly more modern C++ construct.