I believe this is the same problem as
https://gcc.gnu.org/ml/gcc-patches/2008-07/msg00292.html
The asprinf declaration is messed up when using clang to build gdb.
diff --git a/include/libiberty.h b/include/libiberty.h
index b33dd65..a294903 100644
--- a/include/libiberty.h
+++ b/include/libiberty.h
@@ -625,8 +625,10 @@ extern int pwait (int, int *, int);
/* Like sprintf but provides a pointer to malloc'd storage, which must
be freed by the caller. */
+#ifndef asprintf
extern int asprintf (char **, const char *, ...) ATTRIBUTE_PRINTF_2;
#endif
+#endif
/* Like asprintf but allocates memory without fail. This works like
xmalloc. */
