Add __ANDROID_NDK__ to identify using the NDK.

I've added some things like __ANDROID_MAJOR__ to an ndk-version.h, but
that is only in the NDK itself and so doesn't help the platform. Add
__ANDROID_NDK__ to identify that you're building for the NDK and not
the platform.

Test: make checkbuild
Bug: https://github.com/android-ndk/ndk/issues/407
Change-Id: I2d1f1c28e3764e4e658cf675b290b7a17253ee33
diff --git a/libc/include/android/api-level.h b/libc/include/android/api-level.h
index c83c18d..7e3aa99 100644
--- a/libc/include/android/api-level.h
+++ b/libc/include/android/api-level.h
@@ -41,6 +41,8 @@
 
 #ifndef __ANDROID_API__
 #define __ANDROID_API__ __ANDROID_API_FUTURE__
+#else
+#define __ANDROID_NDK__ 1
 #endif
 
 #define __ANDROID_API_G__ 9