configury: set and use HAVE_FDATASYNC for fdatasync support.

FreeBSD 10.x claims POSIX.2001 compliance, but doesn't implement
the fdatasync API.
* configure.ac (AC_CHECK_FUNCS): Add fdatasync.
* ext/posix/unistd.c (Pfdatasync, posix_unistd_fns): Elide
posix.unistd.fdatasync when HAVE_FDATASYNC was not specified,
instead of LPOSIX_2001_COMPLIANT.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>

configure.ac

@@ -128,7 +128,8 @@ case $ac_cv_header_sys_msg_h,$ac_cv_func_msgrcv,$ac_cv_func_msgsnd in
     ;;
 esac
 
-
+## FreeBSD 10.x doesn't have fdatasync.
+AC_CHECK_FUNCS([fdatasync])
 AC_CHECK_DECLS_ONCE([fdatasync])
 
 ## Search libs

ext/posix/unistd.c

@@ -341,7 +341,7 @@ Pexecp(lua_State *L)
 }
 
 
-#if LPOSIX_2001_COMPLIANT
+#if HAVE_FDATASYNC
 
 #if !HAVE_DECL_FDATASYNC
 extern int fdatasync ();
@@ -1048,7 +1048,7 @@ static const luaL_Reg posix_unistd_fns[] =
 	LPOSIX_FUNC( Pdup2		),
 	LPOSIX_FUNC( Pexec		),
 	LPOSIX_FUNC( Pexecp		),
-#if LPOSIX_2001_COMPLIANT
+#if HAVE_FDATASYNC
 	LPOSIX_FUNC( Pfdatasync		),
 #endif
 	LPOSIX_FUNC( Pfork		),