Ticket #2906: crash on Solaris while trying to copy a file.

Ticket #2906: crash on Solaris while trying to copy a file.

Sync with gnulib c25bdbae48977a527dff69150f59fb0746d31b51:
fs usage: check for GNU/Linux statvfs problem dynamically.

* src/filemanager/mountlist.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.
* src/filemanager/filegui.c (filegui__check_attrs_on_fs): apply the same
statvfs_works function.
* m4.include/fsusage.m4 (gl_FILE_SYSTEM_USAGE): Remove static check for
statvfs on GNU/Linux hosts, since it's now done dynamically.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>