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 Signed-off-by: Matt Turner mattst88@gmail.com (cherry picked from commit f475b6fd)