Don't look for sched_yield on Android.

The problem is that sched_yield is available on the target, but configure.ac is asking to build
a host binary using the AC_SEARCH_LIBS macro.  It can't configure for Android, since sched_yield isn't
available on the host.

configure.ac

@@ -190,7 +190,7 @@ AM_CONDITIONAL([HAVE_PTHREAD], [test "x$ax_pthread_ok" = "xyes"])
 AC_CXX_STL_HASH
 
 case "$target_os" in
-  mingw* | cygwin* | win* | aix*)
+  mingw* | cygwin* | win* | aix* | *android* )
     ;;
   *)
     # Need to link against rt on Solaris