Update android ndk to r17c and update documentation

We need to bump the version number for our ndk, otherwise we will get configure errors while building our python recipes with extra libraries. To avoid import errors for python3's ctypes module we must add an extra dependency to our docker file, the libffi-dev package.

Dockerfile

@@ -15,7 +15,7 @@ ENV HOME_DIR="/home/${USER}"
 ENV WORK_DIR="${HOME_DIR}" \
     PATH="${HOME_DIR}/.local/bin:${PATH}"
 # get the latest version from https://developer.android.com/ndk/downloads/index.html
-ENV ANDROID_NDK_VERSION="16b"
+ENV ANDROID_NDK_VERSION="17c"
 # get the latest version from https://www.crystax.net/en/download
 ENV CRYSTAX_NDK_VERSION="10.3.2"
 # get the latest version from https://developer.android.com/studio/index.html
@@ -44,7 +44,7 @@ RUN apt update -qq && apt install -qq --yes --no-install-recommends \
 RUN dpkg --add-architecture i386 && apt update -qq && apt install -qq --yes --no-install-recommends \
     build-essential ccache git libncurses5:i386 libstdc++6:i386 libgtk2.0-0:i386 \
     libpangox-1.0-0:i386 libpangoxft-1.0-0:i386 libidn11:i386 python2.7 python2.7-dev \
-    openjdk-8-jdk zip unzip zlib1g-dev zlib1g:i386
+    libffi-dev openjdk-8-jdk zip unzip zlib1g-dev zlib1g:i386
 
 # specific recipes dependencies (e.g. libffi requires autoreconf binary)
 RUN apt install -qq --yes --no-install-recommends \

doc/source/quickstart.rst

@@ -93,11 +93,34 @@ named ``tools``, and you will need to run extra commands to install
 the SDK packages needed. 
 
 For Android NDK, note that modern releases will only work on a 64-bit
-operating system. If you are using a 32-bit distribution (or hardware),
-the latest useable NDK version is r10e, which can be downloaded here:
+operating system. The minimal, and recommended, NDK version to use is r17c:
+
+ - `Linux (64-bits) <https://dl.google.com/android/repository/android-ndk-r17c-linux-x86_64.zip>`_
+ - `Macos X (64-bits) <https://dl.google.com/android/repository/android-ndk-r17c-darwin-x86_64.zip>`_
+ - Windows users should create a virtual machine with an GNU Linux os
+   installed, and then you can follow the described instructions from within
+   your virtual machine.
+
+.. note::
+    Versions below r17 may cause troubles while compiling some recipes. Our
+    tests had been done against ndk versions r17b and r17c. Later versions of
+    the android ndk still are not proved to work with python-for-android, and
+    may work or not...so...better to stick to the recommended version (r17c).
+
+If you are using a 32-bit distribution (or hardware),
+the latest usable NDK version is r10e, which can be downloaded here:
 
 - `Legacy 32-bit Linux NDK r10e <http://dl.google.com/android/ndk/android-ndk-r10e-linux-x86.bin>`_
 
+.. warning::
+    **32-bit distributions**
+
+    Since the python2 recipe updated to version 2.7.15, the build system has
+    been changed and you should use an old release of python-for-android, which
+    contains the legacy python recipe (v2.7.2). The last python-for-android
+    release with the legacy version of python is version
+    `0.6.0 <https://github.com/kivy/python-for-android/archive/0.6.0.zip>`_.
+
 First, install a platform to target (you can also replace ``19`` with
 a different platform number, this will be used again later)::
 
@@ -113,7 +136,7 @@ Then, you can edit your ``~/.bashrc`` or other favorite shell to include new env
 
     # Adjust the paths!
     export ANDROIDSDK="$HOME/Documents/android-sdk-21"
-    export ANDROIDNDK="$HOME/Documents/android-ndk-r10e"
+    export ANDROIDNDK="$HOME/Documents/android-ndk-r17c"
     export ANDROIDAPI="26"  # Target API version of your application
     export NDKAPI="19"  # Minimum supported API version of your application
     export ANDROIDNDKVER="r10e"  # Version of the NDK you installed