xfree86: Test presence of isastream()

isastream() was never more than a stub in glibc, and was removed in
glibc-2.30 by commit a0a0dc83173c ("Remove obsolete, never-implemented
XSI STREAMS declarations").

Bug: https://bugs.gentoo.org/700838


Reviewed-by: Julien Cristau <jcristau@debian.org>
Signed-off-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit e6ab7f9f)