This tutorial explains how to create Linux -> android-eabi, android-x86
cross compilers and compile the Giac library. The tutorial has been
tested on Kubuntu 14.04, Ubuntu 14.10 and Ubuntu 16.04 systems.

Prerequisites
-------------

1. Download Crystax NDK 10.2.1 from https://crystax.net/en/download. You may need about 2 GB free space.

2. Unpack the contents to ~/android-sdks/. Note that you need about 8 GB extra free space.

3. Set the enviroment variables:
   ```
   NDK_DIR=~/android-sdks/crystax-ndk-10.2.1
   CC_DIR=~/cross-compilers
   ```
4. Set
   ```
   ARCH=arm
   HOST=arm-linux-androideabi
   ```
5. Run
   ```
   $NDK_DIR/build/tools/make-standalone-toolchain.sh --ndk-dir=$NDK_DIR --arch=$ARCH --platform=android-21 --install-dir=$CC_DIR/$ARCH
   ```
6. Type
   ```
   export PATH=$CC_DIR/$ARCH/bin/:$PATH
   ```
7. Optionally check out GMP and MPFR and make them with the following command (for versions see src/jni/prebuilt/android/README.txt):
   ```
    CFLAGS="-fPIC" ./configure --host=$HOST --prefix=$CC_DIR/$ARCH/sysroot/usr --disable-assembly && make && make install
   ```
   This step requires additional 300-500 MB disk space.

8. The static and shared libraries can be found in `$CC_DIR/$ARCH/sysroot/usr/lib`

9. Repeat the steps 4-8 above but instead in step 4 use
   ```
   ARCH=x86
   HOST=i686-linux-android
   ```
   
10. Repeat the steps 4-8 above but instead in step 4 use
    ```
    ARCH=x86_64
    HOST=x86_64-linux-android
    ```
    
11. Repeat the steps 4-8 above but instead in step 4 use
    ```
    ARCH=arm64
    HOST=aarch64-linux-android
    ```

11. Now you may remove the `$NDK_DIR` folder and save some disk space.

Compilation
-----------

1. Make sure that the `PATH` variable is properly set (see step 6 above).

2. Run
   ```
   ../gradlew androidAar
   ```
   in this directory.

Troubleshooting
---------------

You may need to create the file `local.properties` on your own
with `sdk.dir=/path/to/android/sdk` if you don't want to set the
`ANDROID_SDK` variable manually.