Printing warning messages on stderr

	The [[warning()]] function accepts a [[printf]]-style variable
	length argument list, which it prints on [[stderr]].

<<*>>=
#include <config.h>

EXPORT void warning(char *format,...)
{
    va_list args;
    va_start(args, format);
    vfprintf(stderr, format, args);
    va_end(args);
}