mountlist: Replace a configure-time error with a compile-time error.
Reported by Yury V. Zaytsev <yury@shurup.com> in
<https://lists.gnu.org/archive/html/bug-gnulib/2025-03/msg00080.html>.
* m4/mountlist.m4 (gl_MOUNTLIST): Instead of aborting the configuration,
define MOUNTED_NOT_PORTED.
* lib/mountlist.c (read_file_system_list): Err out if MOUNTED_NOT_PORTED
is defined.
* modules/mountlist (configure.ac, Makefile.am): Compile mountlist.c on
all platforms.