fsusage: check for GNU/Linux statvfs problem dynamically

fsusage: check for GNU/Linux statvfs problem dynamically

Problem reported by Nikolaus Rath in <http://bugs.gnu.org/11406>.
* lib/fsusage.c [STAT_STATVFS && __linux__ && (__GLIBC__||__UCLIBC__)]:
Define STAT_STATFS2_BSIZE too, since in this case the code now
checks dynamically whether statvfs is reliable, falling back on
Linux-style statfs otherwise.
(statvfs_works): New function, for dynamically testing statvfs.
(get_fs_usage) [STAT_STATVFS]: Use it.
* m4/fsusage.m4 (gl_FILE_SYSTEM_USAGE): Remove static check for
statvfs on GNU/Linux hosts, since it's now done dynamically.