mkheader inserts #undef if *any* macro is found by the same name, even if in unrelated header

[Jeff Johnson]

Perhaps not a problem, but a little disconcerting when a new add ripples
changes into existing headers.

libncursesw has a *lot* of macros, some with the same name as interfaces
included. In ncursesw/curses.h, similar to the upstream header, an "#undef foo"
is created by mkheader before the "extern foo" appears. What's a little odd is
that these propogate into other headers, for instance stdio.h picked one up:

=== modified file 'headers/stdio.h'
--- headers/stdio.h 2013-01-08 15:55:54 +0000
+++ headers/stdio.h 2011-01-31 15:56:05 +0000
@@ -198,7 +198,6 @@
     extern int asprintf(char **__ptr, const char *__fmt, ...);
     extern int vfscanf(FILE * __s, const char *__format, va_list __arg);
     extern int vscanf(const char *__format, va_list __arg);
-#undef vsscanf
     extern int vsscanf(const char *__s, const char *__format,
                       va_list __arg);
 #endif /* __LSB_VERSION__ >= 1.3 */

Probably doesn't do any harm, but I wonder if mkheader should possibly only
consider the current header and the inclusion chain for macros, before adding
the #undef.