This incorporates:
2020-06-28 getrandom: do not depend on ‘open’ on mingw
2020-06-28 getrandom: fix compilation errors on older versions of mingw
* build-aux/config.sub, lib/getrandom.c, m4/getrandom.m4:
Copy from Gnulib
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.