other/823: gcc -U does not undef a macro

[Richard Earnshaw]

It was this patch that changed the behaviour:

http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/gcc.c.diff?r1=1.162&r2=1.163

However, there is nothing in the ChangeLog for that entry that says why 
this change was introduced.

Zack, was this a mistake?

2000-09-06  Zack Weinberg  <zack@wolery.cumb.org>

	Integrated preprocessor.
	...
	* gcc.c (C specs): Use %(trad_capable_cpp) for -E|-M|-MM case
	#if USE_CPPLIB.