Fix a MinGW warning in libiberty/strerror.c

[Eli Zaretskii <eliz@gnu.org>]

When compiling GDB 7.8.1, I get this warning in libiberty:

     gcc -c -DHAVE_CONFIG_H -O0 -g3 -D__USE_MINGW_ACCESS  -I. -I./../include  -W -Wall -Wwrite-strings -Wc++-compat -Wstrict-prototypes -pedantic  ./strerror.c -o strerror.o
     ./strerror.c:472:12: warning: '_sys_nerr' redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
     ./strerror.c:473:14: warning: '_sys_errlist' redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]

This happens because the MinGW system headers have some special magic
for these variables, which are imported from a system shared library.

The solution I propose is to refrain from declaring variables that are
actually macros, because this should be a sign that something tricky
is going on:

--- libiberty/strerror.c~0	2014-06-11 18:34:41 +0300
+++ libiberty/strerror.c	2014-12-30 08:12:00 +0200
@@ -469,8 +469,13 @@
 
 #else
 
+
+#ifndef sys_nerr
 extern int sys_nerr;
+#endif
+#ifndef sys_errlist
 extern char *sys_errlist[];
+#endif
 
 #endif
 

OK to commit this (with a suitable ChangeLog entry)?