Sync with gnulib 502809019bd2ca3ce3d041d18c35ce9420eedb72.

Avoid hasmntopt const type warning on Solaris.

Solaris defines the OPT param of hasmntopt() with char * instead of
const char *. Passing the constant string "ignore" generates a compiler
warning. For Solaris cast MNT_IGNORE to avoid the warning.

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