Enhancement patch for cpp

[Zack Weinberg]

>> 1999-01-15  Ben Elliston  <bje@cygnus.com>
>>
>>         * cppfiles.c (append_include_chain): Emit a warning if the
>>         directory specified in a -I flag does not exist.
>>
>>         * cccp.c (new_include_prefix): Likewise.

I'm not sure about this patch.  Ignoring nonexistent directories is
done because the default include path has several dirs on it that
often don't exist, such as /usr/local/include and
/usr/cpu-company-system/include.  They are processed through the same
function, and warning about them would be confusing.

zw