small fixes to headers for non-__GCC__ compilers.

[Mike McCabe]

(Using egcs-2-91.66, glibc-2.1.92-14; please point me properly or forward if
this belongs elsewhere.)

Looks like the gcc header files are slightly incorrect for non- __GCC__
compilers.

I compiled and ran lcc on a simple program that used stdio.h, and
encountered the following problems:

malloc.h:79 and sys/cdefs.h:54 both need #define __THROW <nothing> in the
non-__GCC__ clause.
gconv.h:172 - need to make use of gcc zero-length array extension #ifdef
__GCC__.

Thanks - I'll reply to this message if I find more similar problems.

Mike