Restore __ANDROID_NDK__.

At heart a revert of "Don't #define __ANDROID_NDK__ for the platform build!",
which was commit eb6143037423a241ba8035304936149b300f7a5d.

The original change was insufficiently motivated, and meant that the
NDK -- not just the platform -- no longer defines __ANDROID_NDK__. Which
then broke at least building toybox with NDK r19.

Change-Id: Ic616688e4d17d25714a9ef381269d7431deac9b0

1 files changed, 39 insertions, 20 deletions

diff --git a/libc/include/android/api-level.h b/libc/include/android/api-level.h
index af7cde1e3..4a3d05207 100644
--- a/libc/include/android/api-level.h
+++ b/libc/include/android/api-level.h
@@ -37,70 +37,89 @@
 
 __BEGIN_DECLS
 
-#ifndef __ANDROID_API_FUTURE__
 /**
  * Magic version number for an Android OS build which has
  * not yet turned into an official release,
- * for comparisons against __ANDROID_API__.
+ * for comparison against `__ANDROID_API__`.
  */
 #define __ANDROID_API_FUTURE__ 10000
-#endif
 
+/* This #ifndef should never be true except when doxygen is generating docs. */
 #ifndef __ANDROID_API__
 /**
  * `__ANDROID_API__` is the API level being targeted. For the OS,
  * this is `__ANDROID_API_FUTURE__`. For the NDK, this is set by the
- * compiler/build system based on the API level you claimed to target.
+ * compiler system based on the API level you claimed to target.
  */
 #define __ANDROID_API__ __ANDROID_API_FUTURE__
 #endif
 
-/** Names the Gingerbread API level (9), for comparisons against __ANDROID_API__. */
+#if __ANDROID_API__ != __ANDROID_API_FUTURE__
+/**
+ * `__ANDROID_NDK__` is defined for code that's built by the NDK
+ * rather than as part of the OS. "Built by the NDK" is an ambiguous idea,
+ * so you might prefer to check `__ANDROID__`, `__BIONIC__`, `__linux__`,
+ * or `__NDK_MAJOR__` instead, depending on exactly what you're trying to say.
+ *
+ * `__ANDROID_NDK__` is intended to capture "this code is being built for
+ * Android, but targeting a specific API level". This is true for all code built
+ * by the NDK proper, but also for OS code that sets `sdk_version` in its build
+ * file. This distinction might matter to you if, for example, your code could
+ * be built as part of an app *or* as part of the OS.
+ */
+#define __ANDROID_NDK__ 1
+#endif
+
+/** Names the Gingerbread API level (9), for comparison against `__ANDROID_API__`. */
 #define __ANDROID_API_G__ 9
 
-/** Names the Ice-Cream Sandwich API level (14), for comparisons against __ANDROID_API__. */
+/** Names the Ice-Cream Sandwich API level (14), for comparison against `__ANDROID_API__`. */
 #define __ANDROID_API_I__ 14
 
-/** Names the Jellybean API level (16), for comparisons against __ANDROID_API__. */
+/** Names the Jellybean API level (16), for comparison against `__ANDROID_API__`. */
 #define __ANDROID_API_J__ 16
 
-/** Names the Jellybean MR1 API level (17), for comparisons against __ANDROID_API__. */
+/** Names the Jellybean MR1 API level (17), for comparison against `__ANDROID_API__`. */
 #define __ANDROID_API_J_MR1__ 17
 
-/** Names the Jellybean MR2 API level (18), for comparisons against __ANDROID_API__. */
+/** Names the Jellybean MR2 API level (18), for comparison against `__ANDROID_API__`. */
 #define __ANDROID_API_J_MR2__ 18
 
-/** Names the KitKat API level (19), for comparisons against __ANDROID_API__. */
+/** Names the KitKat API level (19), for comparison against `__ANDROID_API__`. */
 #define __ANDROID_API_K__ 19
 
-/** Names the Lollipop API level (21), for comparisons against __ANDROID_API__. */
+/** Names the Lollipop API level (21), for comparison against `__ANDROID_API__`. */
 #define __ANDROID_API_L__ 21
 
-/** Names the Lollipop MR1 API level (22), for comparisons against __ANDROID_API__. */
+/** Names the Lollipop MR1 API level (22), for comparison against `__ANDROID_API__`. */
 #define __ANDROID_API_L_MR1__ 22
 
-/** Names the Marshmallow API level (23), for comparisons against __ANDROID_API__. */
+/** Names the Marshmallow API level (23), for comparison against `__ANDROID_API__`. */
 #define __ANDROID_API_M__ 23
 
-/** Names the Nougat API level (24), for comparisons against __ANDROID_API__. */
+/** Names the Nougat API level (24), for comparison against `__ANDROID_API__`. */
 #define __ANDROID_API_N__ 24
 
-/** Names the Nougat MR1 API level (25), for comparisons against __ANDROID_API__. */
+/** Names the Nougat MR1 API level (25), for comparison against `__ANDROID_API__`. */
 #define __ANDROID_API_N_MR1__ 25
 
-/** Names the Oreo API level (26), for comparisons against __ANDROID_API__. */
+/** Names the Oreo API level (26), for comparison against `__ANDROID_API__`. */
 #define __ANDROID_API_O__ 26
 
-/** Names the Oreo MR1 API level (27), for comparisons against __ANDROID_API__. */
+/** Names the Oreo MR1 API level (27), for comparison against `__ANDROID_API__`. */
 #define __ANDROID_API_O_MR1__ 27
 
-/** Names the Pie API level (28), for comparisons against __ANDROID_API__. */
+/** Names the Pie API level (28), for comparison against `__ANDROID_API__`. */
 #define __ANDROID_API_P__ 28
 
-/** Names the "Q" API level (29), for comparisons against __ANDROID_API__. */
+/**
+ * Names the "Q" API level (29), for comparison against `__ANDROID_API__`.
+ * This release was called Android 10 publicly, not to be (but sure to be)
+ * confused with API level 10.
+ */
 #define __ANDROID_API_Q__ 29
 
-/** Names the "R" API level (30), for comparisons against __ANDROID_API__. */
+/** Names the "R" API level (30), for comparison against `__ANDROID_API__`. */
 #define __ANDROID_API_R__ 30
 
 /**