a68: use INCLUDE_FOO before system.h for standard C++ headers

Define INCLUDE_* preprocessor symbols rather than including some
standard C++ headers directly.

Signed-off-by: Jose E. Marchesi <jemarch@gnu.org>

gcc/algol68/ChangeLog

	* a68-parser-brackets.cc (INCLUDE_STRING): Define.
	Do not include <string.
	* a68-parser-bottom-up.cc (INCLUDE_STRING): Define.
	Do not include <string>.
	* a68-moids-diagnostics.cc (INCLUDE_STRING): Define.
	Do not include <string>.
	* a68-imports.cc (INCLUDE_STRING): Define.
	Do not include <string>.
	* a68-imports-archive.cc (INCLUDE_MAP): Define.
	(INCLUDE_STRING): Likewise.
	Do not include <string> nor <map>.