Check Android build on Travis CI.

Also stop requiring sudo privilege so that we can use containers
for testing.

.travis.yml

@@ -7,7 +7,7 @@ os:
   - osx
 
 # Use Ubuntu 14.04 LTS (Trusty) as the Linux testing environment.
-sudo: required
+sudo: false
 dist: trusty
 
 env:
@@ -24,6 +24,9 @@ matrix:
     # Skip GCC builds on Mac OS X.
     - os: osx
       compiler: gcc
+  include:
+    # Additional build using Android NDK.
+    - env: BUILD_NDK=ON
 
 cache:
   apt: true
@@ -41,20 +44,38 @@ addons:
 install:
   # Install ninja on Mac OS X.
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update && brew install ninja; fi
-  # Make sure that clang-3.6 is selected.
+  # Make sure that clang-3.6 is selected on Linux.
   - if [[ "$TRAVIS_OS_NAME" == "linux" && "$CC" == "clang" ]]; then
       export CC=clang-3.6 CXX=clang++-3.6;
     fi
+  # Download Android NDK and Android CMake toolchain file.
+  - if [[ "$BUILD_NDK" == "ON" ]]; then
+      git clone --depth=1 https://github.com/urho3d/android-ndk.git $HOME/android-ndk;
+      export ANDROID_NDK=$HOME/android-ndk;
+      git clone --depth=1 https://github.com/taka-no-me/android-cmake.git $HOME/android-cmake;
+      export TOOLCHAIN_PATH=$HOME/android-cmake/android.toolchain.cmake;
+    fi
 
 before_script:
   - git clone https://github.com/google/googletest.git External/googletest
 
 script:
   - mkdir build && cd build
-  # We need to install the compiled binaries so the paths in the runtests script can resolve correctly.
-  - cmake -GNinja -DCMAKE_BUILD_TYPE=${GLSLANG_BUILD_TYPE} -DCMAKE_INSTALL_PREFIX=`pwd`/install ..
-  - ninja install
-  # Run Google-Test-based tests.
-  - ctest --output-on-failure
-  # Run runtests-based tests.
-  - cd ../Test && ./runtests
+  # For Android, do release building using NDK without testing.
+  # For Linux and macOS, do debug/release building with testing.
+  - if [[ "$BUILD_NDK" == "ON" ]]; then
+      cmake -DCMAKE_TOOLCHAIN_FILE=${TOOLCHAIN_PATH}
+            -DANDROID_NATIVE_API_LEVEL=android-12
+            -DCMAKE_BUILD_TYPE=Release
+            -DANDROID_ABI="armeabi-v7a with NEON"
+            -DBUILD_TESTING=OFF
+            -GNinja ..;
+      ninja;
+    else
+      cmake -DCMAKE_BUILD_TYPE=${GLSLANG_BUILD_TYPE}
+            -DCMAKE_INSTALL_PREFIX=`pwd`/install
+            -GNinja ..;
+      ninja install;
+      ctest --output-on-failure &&
+      cd ../Test && ./runtests;
+    fi