Work around lack of flock(2) declaration on Tru64 5.1b.

* configure.in: Check for the declaration of flock(2).

* libguile/posix.c [!__MINGW32__][!HAVE_DECL_FLOCK]: Provide an flock(2)
  declaration, needed on Tru64 5.1b.

configure.in

@@ -774,10 +774,12 @@ AC_CHECK_FUNCS([DINFINITY DQNAN cexp chsize clog clog10 ctermid fesetround ftime
 #       declaration isn't anywhere.
 #   cuserid - on Tru64 5.1b the declaration is documented to be available
 #       only with `_XOPEN_SOURCE' or some such.
+#   flock - on Tru64 5.1b the declaration is available from <sys/fcntl.h>
+#       but only if `_BSD' is defined.
 #
 AC_CHECK_HEADERS(crypt.h netdb.h pthread.h sys/param.h sys/resource.h sys/file.h)
 AC_CHECK_FUNCS(chroot flock getlogin cuserid getpriority setpriority getpass sethostname gethostname)
-AC_CHECK_DECLS([sethostname, strncasecmp, unsetenv, hstrerror, cuserid])
+AC_CHECK_DECLS([sethostname, strncasecmp, unsetenv, hstrerror, cuserid, flock])
 
 # crypt() may or may not be available, for instance in some countries there
 # are restrictions on cryptography.

libguile/posix.c

@@ -1856,6 +1856,13 @@ static int flock (int fd, int operation)
 #endif /* __MINGW32__ */
 
 #if HAVE_FLOCK || defined (__MINGW32__)
+
+#ifndef __MINGW32__
+# if !HAVE_DECL_FLOCK
+extern int flock (int, int);
+# endif
+#endif
+
 SCM_DEFINE (scm_flock, "flock", 2, 0, 0, 
             (SCM file, SCM operation),
 	    "Apply or remove an advisory lock on an open file.\n"