Forward-compatible fix for ucontext_t

The Android C library <signal.h> will soon be modified to declare
ucontext_t in <signal.h>. To avoid conflicts when this happens, only
define the type when needed.

See b/issue?id=2189142

Change-Id: I297b8e6f9fba8fb99c5f799b0d7e7f4dbda914bb
diff --git a/base/ b/base/
index 3270079..a741892 100644
--- a/base/
+++ b/base/
@@ -39,8 +39,8 @@
 extern char** environ;
-#ifdef ANDROID
-// No ucontext.h on Android
+#if defined(__ANDROID__) && !defined(__BIONIC_HAVE_UCONTEXT_T)
+// No ucontext.h on old Android C library headers
 typedef void ucontext_t;