Check for and compile using -std=gnu11 (or higher)

Check for and compile using -std=gnu11 (or higher)

Use gnulib m4/std-gnu11.m4 for that.

Also update the minimum autoconf version to 2.69. And the minimum
automake to 1.13. This lets us clean up configure.ac a little.

We would really like 2.70+ so we don't need the gnulib gnu11.m4. But
2.69 still seems in use on various stable systems. autoconf 2.70
was released end of 2020, autoconf 2.69 in 2012.