Update Android build instructions.

We now have a copy of android-cmake. Also remove the mention of running cmake
twice. It seems to work fine once?

The API level also got specified twice somehow.

BUG=26

Change-Id: I1331b079a4d8531cd53f7de3605ac318c14b3e26
Reviewed-on: https://boringssl-review.googlesource.com/7985
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/BUILDING.md b/BUILDING.md
index 33ad87a..0e34db3 100644
--- a/BUILDING.md
+++ b/BUILDING.md
@@ -79,18 +79,18 @@
 been tested with version 10d of the NDK.
 
 Unpack the Android NDK somewhere and export `ANDROID_NDK` to point to the
-directory. Clone https://github.com/taka-no-me/android-cmake into `util/`.  Then
-make a build directory as above and run CMake *twice* like this:
+directory. Then make a build directory as above and run CMake like this:
 
-    cmake -DANDROID_NATIVE_API_LEVEL=android-9 \
-          -DANDROID_ABI=armeabi-v7a \
-          -DCMAKE_TOOLCHAIN_FILE=../util/android-cmake/android.toolchain.cmake \
+    cmake -DANDROID_ABI=armeabi-v7a \
+          -DCMAKE_TOOLCHAIN_FILE=../third_party/android-cmake/android.toolchain.cmake \
           -DANDROID_NATIVE_API_LEVEL=16 \
           -GNinja ..
 
-Once you've run that twice, Ninja should produce Android-compatible binaries.
-You can replace `armeabi-v7a` in the above with `arm64-v8a` to build aarch64
-binaries.
+Once you've run that, Ninja should produce Android-compatible binaries.  You
+can replace `armeabi-v7a` in the above with `arm64-v8a` and use API level 21 or
+higher to build aarch64 binaries.
+
+For other options, see [android-cmake's documentation](./third_party/android-cmake/README.md).
 
 ## Known Limitations on Windows