getrandom: do not depend on ‘open’ on mingw
Similarly for at-internal, getloadavg. These modules do not call
the ‘open’ function when they are compiled on mingw. On mingw,
this avoids having to compile open.c when building Emacs, which
does its own thing with ‘open’.
* modules/at-internal, modules/getloadavg, modules/getrandom:
(Depends-on): Don’t depend on ‘open’ on mingw.
(Depends-on): Require AC_CANONICAL_HOST, for host_os.
* modules/getloadavg (Depends-on):
Depend on intprops, open, stdbool, stdlib only if compiling
getloadavg.c.