gfortran.texi: Mention that asynchronous I/O does not work on systems which lack...

gfortran.texi: Mention that asynchronous I/O does not work on systems which lack condition...

2018-08-22  Thomas Koenig  <tkoenig@gcc.gnu.org>

	* gfortran.texi: Mention that asynchronous I/O does
	not work on systems which lack condition variables, such
	as AIX.

2018-08-22  Thomas Koenig  <tkoenig@gcc.gnu.org>

	* async.h: Set ASYNC_IO to zero if _AIX is defined.
	(struct adv_cond): If ASYNC_IO is zero, the struct has no members.
	(async_unit): If ASYNC_IO is zero, remove unneeded members.

From-SVN: r263788