Update from Gnulib

Update from Gnulib

This incorporates:
2019-03-10 alloca-opt: Fix conflict mingw's new <alloca.h> file
2019-03-03 getloadavg: Write NULL for the null pointer
Reported by Michal Privoznik <mprivozn@redhat.com>.
* lib/getloadavg.c (getloadavg): Write NULL instead of 0.
* build-aux/config.guess, build-aux/move-if-change:
* doc/misc/texinfo.tex, lib/alloca.in.h, lib/getloadavg.c:
* m4/alloca.m4:
Copy from Gnulib.
* lib/gnulib.mk.in: Regenerate.