Add android build step

.travis.yml

@@ -51,3 +51,32 @@ matrix:
       script:
         - ./scripts/cargo_test.sh
       cache: cargo
+    - language: rust
+      dist: trusty
+      rust: stable
+      jdk:
+        - oraclejdk8
+      env:
+        # Get revision number from https://dl.google.com/android/repository/repository2-1.xml
+        # Look for the string 'sdk-tools-linux-'.
+        - ANDROID_SDK_VERSION=4333796
+
+      before_install:
+        - >
+          PKG_OS="$TRAVIS_OS_NAME";
+          export ANDROID_HOME="${HOME}/android-sdk";
+          export PATH=$PATH:"${ANDROID_HOME}/tools/bin";
+          mkdir $ANDROID_HOME
+      install:
+        # SDK Tools.
+        - >
+          curl -o sdk.zip "https://dl.google.com/android/repository/sdk-tools-${PKG_OS}-${ANDROID_SDK_VERSION}.zip" &&
+          unzip -q sdk.zip -d $ANDROID_HOME;  > /dev/null
+        # Android SDK and NDK
+        # Unfortunately we need to get rid of all output, otherwise it can easily fill up the 4 MB log limit
+        - yes | sdkmanager --licenses > /dev/null
+        - sdkmanager 'platform-tools' 'build-tools;27.0.0' 'platforms;android-27' 'ndk-bundle' > /dev/null
+      script:
+        - cd sdks/android/Mentat
+        - ./gradlew --no-daemon clean :library:assemble :library:test :library:lint
+        - cd ../../..

cargo-config.toml.template

@@ -0,0 +1,11 @@
+[target.aarch64-linux-android]
+ar = "$PWD/NDK/arm64/bin/aarch64-linux-android-ar"
+linker = "$PWD/NDK/arm64/bin/aarch64-linux-android-clang"
+
+[target.armv7-linux-androideabi]
+ar = "$PWD/NDK/arm/bin/arm-linux-androideabi-ar"
+linker = "$PWD/NDK/arm/bin/arm-linux-androideabi-clang"
+
+[target.i686-linux-android]
+ar = "$PWD/NDK/x86/bin/i686-linux-android-ar"
+linker = "$PWD/NDK/x86/bin/i686-linux-android-clang"

scripts/create_standalone_ndk.sh

@@ -0,0 +1,45 @@
+#!/bin/sh
+
+set -eu
+
+if [ -d NDK ]; then
+    printf '\033[33;1mStandalone NDK already exists... Delete the NDK folder to make a new one.\033[0m\n\n'
+    printf '  $ rm -rf NDK\n'
+    exit 0
+fi
+
+if [ ! -d "${ANDROID_SDK_ROOT-}" ]; then
+    ANDROID_SDK_ROOT=/usr/local/share/android-sdk
+fi
+if [ ! -d "${ANDROID_HOME-}" ]; then
+    ANDROID_HOME="$ANDROID_SDK_ROOT"
+fi
+if [ ! -d "${ANDROID_NDK_HOME-}" ]; then
+    ANDROID_NDK_HOME="$ANDROID_HOME/ndk-bundle"
+fi
+MAKER="${ANDROID_NDK_HOME}/build/tools/make_standalone_toolchain.py"
+
+if [ -x "$MAKER" ]; then
+    echo 'Creating standalone NDK...'
+else
+    printf '\033[91;1mPlease install Android NDK!\033[0m\n\n'
+    printf '  $ sdkmanager ndk-bundle\n\n'
+    printf "\033[33;1mnote\033[0m: file \033[34;4m$MAKER\033[0m not found.\n"
+    printf 'If you have installed the NDK in non-standard location, please define the \033[1m$ANDROID_NDK_HOME\033[0m variable.\n'
+    exit 1
+fi
+
+mkdir NDK
+
+create_ndk() {
+    echo "($1)..."
+    "$MAKER" --api "$2" --arch "$1" --install-dir "NDK/$1"
+}
+
+create_ndk arm64 24
+create_ndk arm 24
+create_ndk x86 24
+
+echo 'Updating cargo-config.toml...'
+
+sed 's|$PWD|'"${PWD}"'|g' cargo-config.toml.template > cargo-config.toml