Our libiberty does not have filename_cmp(), but since it would simply
call strcmp(), use a macro when building gdb and most of all libbfd.

Fixes linking against libbfd with our system libiberty.

--- libiberty/filename_cmp.c.orig	Thu Feb 19 12:58:08 2015
+++ libiberty/filename_cmp.c	Fri Feb 20 22:12:26 2015
@@ -52,6 +52,7 @@ and backward slashes are equal.
 
 */
 
+#if 0
 int
 filename_cmp (const char *s1, const char *s2)
 {
@@ -146,6 +147,7 @@ filename_ncmp (const char *s1, const char *s2, size_t 
   return 0;
 #endif
 }
+#endif
 
 /*