FreeBSD does not need extern char *sys_errlist[]

Add preprocessor directives not to try
to redefine sys_errlist[] or sys_nerr

There are already definitions:

 extern __const char *__const sys_errlist[];
 extern __const int sys_nerr;

in <stdio.h>

Actually we should have something like
NeedSysErrlist in imake definitions
to get rid of those #ifdefs.

cde/programs/dtprintinfo/libUI/MotifUI/Debug.c

@@ -105,7 +105,7 @@ SysErrorMsg(
    int n
    )
 {
-#if !defined(linux)
+#if !defined(linux) && !defined(__FreeBSD__)
     extern char *sys_errlist[];
     extern int sys_nerr;
 #endif