`cgroup2_mount' in the nproc module expects `setmntent' to be present whenever mntent.h is available, but the header itself does not define `setmntent' on releases predating SDK 21, yielding such an error when compiling Emacs:
nproc.c:391:15: error: implicit declaration of function 'setmntent' is invalid
in C99 [-Werror,-Wimplicit-function-declaration]
if (! (fp = setmntent ("/proc/mounts", "r")))
^
