gfortran.h (async_io_dt): Add external reference.

gfortran.h (async_io_dt): Add external reference.

2017-10-07  Thomas Koenig  <tkoenig@gcc.gnu.org>

	* gfortran.h (async_io_dt): Add external reference.
	* io.c (async_io_dt): Add variable.
	(compare_to_allowed_values): Add prototyte. Add optional argument
	num. If present, set it to the number of the entry that was
	matched.
	(check_io_constraints): If this is for an asynchronous I/O
	statement, set async_io_dt and set the asynchronous flag for
	a SIZE tag.
	* resolve.c (resolve_transfer): If async_io_dt is set, set
	the asynchronous flag on the variable.
	(resolve_fl_namelist): If async_io_dt is set, set the asynchronous
	flag on all elements of the namelist.

From-SVN: r253508