mescc: Basic printf support.

mescc: Basic printf support.

* module/mes/libc-i386.mes (i386:va-arg): New function.
* module/mes/libc-i386.scm (mes): Export it.
* module/mes/libc.mes (printf): New function.
  (libc): Add it.
* libc/include/stdio.h: New file.
* module/language/c99/compiler.mes (c99-input->ast): Add libc/include
  to include path.
  (ast-info): Handle (skip) ellipsis in function declaration.