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/dtappbuilder/src/libABil/bil_loadfile.c

@@ -61,7 +61,7 @@
 
 BIL_LOAD_INFO	bilP_load;
 char		Buf[MAXPATHLEN];
-#if !defined(linux)
+#if !defined(linux) && !defined(__FreeBSD__)
 extern char     *sys_errlist[];
 #endif