strerror.c: Do not declare sys_nerr or sys_errlist if already macros

This fixes a MinGW warning in libiberty/strerror.c.

2015-01-19  Eli Zaretskii  <eliz@gnu.org>

        * strerror.c <sys_nerr, sys_errlist>: Declare only if they aren't
        macros.

libiberty/ChangeLog

@@ -1,3 +1,8 @@
+2015-01-19  Eli Zaretskii  <eliz@gnu.org>
+
+	* strerror.c <sys_nerr, sys_errlist>: Declare only if they aren't
+	macros.
+
 2014-12-24  Uros Bizjak  <ubizjak@gmail.com>
 	    Ben Elliston  <bje@au.ibm.com>
 	    Manuel Lopez-Ibanez  <manu@gcc.gnu.org>

libiberty/strerror.c

@@ -469,8 +469,13 @@ static const char **sys_errlist;
 
 #else
 
+
+#ifndef sys_nerr
 extern int sys_nerr;
+#endif
+#ifndef sys_errlist
 extern char *sys_errlist[];
+#endif
 
 #endif