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.

Index: include/filenames.h
--- include/filenames.h.orig
+++ include/filenames.h
@@ -80,11 +80,10 @@ extern "C" {
   (IS_DIR_SEPARATOR_1 (dos_based, (f)[0])		 \
    || HAS_DRIVE_SPEC_1 (dos_based, f))
 
-extern int filename_cmp (const char *s1, const char *s2);
-#define FILENAME_CMP(s1, s2)	filename_cmp(s1, s2)
+#define FILENAME_CMP(s1, s2)		strcmp(s1, s2)
+#define filename_cmp(s1, s2)		strcmp(s1, s2)
 
-extern int filename_ncmp (const char *s1, const char *s2,
-			  size_t n);
+#define filename_ncmp(s1, s2, n)	strncmp(s1, s2, n)
 
 extern hashval_t filename_hash (const void *s);