arch/async_posix.h: improve portability.

{make|swap|get|set}context are removed in POSIX.1-2008, but glibc
apparently keeps providing it.
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7420)

(cherry picked from commit 9d71a24ebf57e7157888af1ca587eafe914bf96f)

crypto/async/arch/async_posix.h

@@ -17,7 +17,8 @@
 
 # include <unistd.h>
 
-# if _POSIX_VERSION >= 200112L
+# if _POSIX_VERSION >= 200112L \
+     && (_POSIX_VERSION < 200809L || defined(__GLIBC__))
 
 # include <pthread.h>