Explain the need to set ANDROID_NDK_HOME env var (#2100)

development/compiling/compiling_for_android.rst

@@ -55,7 +55,10 @@ with ``y``:
 
 
 Set the environment variable ``ANDROID_NDK_ROOT`` to point to the
-Android NDK.
+Android NDK. You also might need to set the variable ``ANDROID_NDK_HOME``
+to the same path, especially if you are using custom Android modules,
+since some Gradle plugins rely on the NDK and use this variable to
+determine its location.
 
 To set those environment variables on Windows, press **Windows + R**, type
 "control system", then click on **Advanced system settings** in the left