grep: S_ISSOCK not POSIX

[Ilya N. Golubev]

Version: 2.5
System: i586-pc-sco3.2v5.0.2

Some buggy libc's have `mbrtowc', but not `mbsinit' (violate Amendment
1 of ISO C90?).  It appears that this is because `mbstate_t' value is
never used by `mbrtowc' (`mbstate_t' is `char', just to shut
compiler).

The fix is to pretend that `mbsinit' always returns true.  A
workaround available to package builder without changing sources is an
addition to `CPPFLAGS' doing that.  The true fix is check for
`mbsinit' function (with all system headers that might define the
macro included) and, if it is not present, `#define' it as above.