WIP: document and enable building for Android

Currently this fails because of an Android incompatibility in glutin:
rust-windowing/glutin#1307
rust-windowing/glutin#1313

Building-Android.md

@@ -0,0 +1,58 @@
+# Building Leafish for Android
+
+This guide assumes a Linux host using glibc (most distributions).
+
+## Install the Android NDK
+
+You can either download the NDK directly, or use the Android SDK tools to manage your NDK's.
+The latter has the benefit of being able to keep your NDK up-to-date relatively easy.
+
+### Download the NDK directly
+
+The NDK can be downloaded from [the Android developer site](https://developer.android.com/ndk/downloads/).
+Download the Linux package and unzip it somewhere, we'll use `~/.android` as the target directory in this guide.
+
+The eventual path will then be `~/.android/android-ndk-r<version>`.
+To finish up, export that path as `$NDK_HOME`, so for example `export NDK_HOME=~/.android/android-ndk-r23`.
+
+### Android SDK tools
+
+Download either Android Studio or the Android command line tools from [here](https://developer.android.com/studio/#downloads).
+Since you probably won't use the editor, this guide will use the command line tools.
+
+Create a directory `~/.android/cmdline-tools` and unzip the downloaded archive to there.
+Rename the unzip directory to `latest`, so the ending path will be `~/.android/cmdline-tools/latest` which includes the `bin` and `lib` directories.
+
+Use the sdkmanager to install the `ndk-bundle`:
+
+```sh
+~/.android/cmdline-tools/latest/bin/sdkmanager install ndk-bundle
+```
+
+To finish up, export the installed NDK as `$NDK_HOME`, so for example `export NDK_HOME=~/.android/ndk-bundle/`.
+
+### Configure Rust and Cargo
+
+Using `rustup`, install the Android target for which you want to build:
+
+```sh
+rustup target add armv7-linux-android aarch64-linux-android i686-linux-android
+```
+
+Then we have to tell Cargo what cross-compiler to use depending on what platform we want to target.
+For example for a x86 Android device:
+
+```sh
+export CC="$NDK_HOME/toolchains/llvm/prebuilt/linux-x86_64/bin/i686-linux-android30-clang"
+```
+
+When building for `aarch64` replace `i686` for `aarch64`, and for `armv7` replace it for `armv7a`.
+
+### Building
+
+Invoke Cargo as usual but with the `--target=<target>-linux-android` argument added.
+For example:
+
+```sh
+cargo build --target=i686-linux-android
+```

Cargo.toml

@@ -74,4 +74,13 @@ version = "0"
 
 [dependencies.leafish_protocol]
 path = "./protocol"
-version = "0"
+version = "0"
+
+[target.i686-linux-android.dependencies]
+openssl = { version = "*", features = ["vendored"] }
+
+[target.armv7a-linux-android.dependencies]
+openssl = { version = "*", features = ["vendored"] }
+
+[target.aarch64-linux-android.dependencies]
+openssl = { version = "*", features = ["vendored"] }